Fix panic on request failure

[pgaskin]

Description

This PR fixes a panic when http.Client.Do returns an error. This issue is caused by trying to close the request body with defer before checking for an error (see golang/go#17780).

I noticed this issue when running dnscontrol (PR StackExchange/dnscontrol#529) on my CI server, and I forgot to install ca-certificates in the image (resulting in the error: Get https://api.vultr.com/v1/account/info: x509: certificate signed by unknown authority).

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x40 pc=0xd277ae]
goroutine 1 [running]:
github.com/StackExchange/dnscontrol/vendor/github.com/vultr/govultr.(*Client).DoWithContext(0xc0006e4680, 0x11d8e60, 0xc0000b4030, 0xc0001c6800, 0xdd0d00, 0xc000295480, 0x0, 0x0)
/home/patrick/go/src/github.com/StackExchange/dnscontrol/vendor/github.com/vultr/govultr/govultr.go:169 +0x12e
github.com/StackExchange/dnscontrol/vendor/github.com/vultr/govultr.(*AccountServiceHandler).GetInfo(0xc0000b8938, 0x11d8e60, 0xc0000b4030, 0xc0006e4680, 0xc0006ef4c8, 0xe9e820)
/home/patrick/go/src/github.com/StackExchange/dnscontrol/vendor/github.com/vultr/govultr/account.go:38 +0xfc
github.com/StackExchange/dnscontrol/providers/vultr.NewProvider(0xc0006d5440, 0x0, 0x0, 0x0, 0xc0000f53d8, 0x1, 0xc0006e6750, 0xeefcc0)
/home/patrick/go/src/github.com/StackExchange/dnscontrol/providers/vultr/vultrProvider.go:64 +0xdc
github.com/StackExchange/dnscontrol/providers.CreateDNSProvider(0xc00003dc35, 0x5, 0xc0006d5440, 0x0, 0x0, 0x0, 0x0, 0x6, 0x63a2e0e200000000, 0xc000627358)
/home/patrick/go/src/github.com/StackExchange/dnscontrol/providers/providers.go:72 +0x92
github.com/StackExchange/dnscontrol/commands.InitializeProviders(0x10076f8, 0xa, 0xc0006c5020, 0x0, 0x11cf480, 0xc000241e20, 0x0, 0x0)
/home/patrick/go/src/github.com/StackExchange/dnscontrol/commands/previewPush.go:219 +0x6c0
github.com/StackExchange/dnscontrol/commands.run(0x100943b, 0xc, 0x0, 0x0, 0x0, 0x0, 0x0, 0x10076f8, 0xa, 0x0, ...)
/home/patrick/go/src/github.com/StackExchange/dnscontrol/commands/previewPush.go:107 +0xf7
github.com/StackExchange/dnscontrol/commands.Preview(...)
/home/patrick/go/src/github.com/StackExchange/dnscontrol/commands/previewPush.go:87
github.com/StackExchange/dnscontrol/commands.glob..func4.1(0xc0000e71e0, 0xc0001a2f00, 0xc0000e71e0)
/home/patrick/go/src/github.com/StackExchange/dnscontrol/commands/previewPush.go:25 +0x97
github.com/StackExchange/dnscontrol/vendor/github.com/urfave/cli.HandleAction(0xe219a0, 0xc0001645d0, 0xc0000e71e0, 0x0, 0xc0001a2f60)
/home/patrick/go/src/github.com/StackExchange/dnscontrol/vendor/github.com/urfave/cli/app.go:501 +0xc8
github.com/StackExchange/dnscontrol/vendor/github.com/urfave/cli.Command.Run(0x1004b97, 0x7, 0x0, 0x0, 0x0, 0x0, 0x0, 0x10334e9, 0x4e, 0x0, ...)
/home/patrick/go/src/github.com/StackExchange/dnscontrol/vendor/github.com/urfave/cli/command.go:165 +0x487
github.com/StackExchange/dnscontrol/vendor/github.com/urfave/cli.(*App).Run(0xc0001728c0, 0xc0000aa020, 0x2, 0x2, 0x0, 0x0)
/home/patrick/go/src/github.com/StackExchange/dnscontrol/vendor/github.com/urfave/cli/app.go:259 +0x6e3
github.com/StackExchange/dnscontrol/commands.Run(0xc000096600, 0x55, 0xc000096058)
/home/patrick/go/src/github.com/StackExchange/dnscontrol/commands/commands.go:58 +0x25d
main.main()
/home/patrick/go/src/github.com/StackExchange/dnscontrol/main.go:18 +0x41 

Checklist:

• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you linted your code locally prior to submission?
• Have you successfully ran tests with your changes locally?

[codecov]

Codecov Report

Merging #20 into master will increase coverage by 0.08%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #20      +/-   ##
==========================================
+ Coverage    71.7%   71.79%   +0.08%     
==========================================
  Files          22       22              
  Lines        2269     2269              
==========================================
+ Hits         1627     1629       +2     
+ Misses        323      322       -1     
+ Partials      319      318       -1

Impacted Files	Coverage Δ
govultr.go	88.23% <100%> (+2.35%)	⬆️

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[ddymko]

@geek1011

Thanks for submitting this bug fix! We really appreciate the PR & fix....we had just noticed this issue as well and we were going to patch it but you beat us to it!