provider/github Check for potentially nil response from GitHub API client

[ewbankkit]

The GitHub API client methods usually return 3 values - object, resp, err.
If a non-nil err is returned, object will be nil but resp may or may not be nil.
In a few places in the GitHub provider there were assumptions that when err != nil, resp != nil and I hit this when I had an invalid GitHub token at one point and got a nil-pointer dereference panic.

Acceptance tests:

make testacc TEST=./builtin/providers/github/ TESTARGS='-run=TestAccGithubIssueLabel_'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/05/19 15:30:58 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/github/ -v -run=TestAccGithubIssueLabel_ -timeout 120m
=== RUN   TestAccGithubIssueLabel_basic
--- PASS: TestAccGithubIssueLabel_basic (2.32s)
=== RUN   TestAccGithubIssueLabel_existingLabel
--- PASS: TestAccGithubIssueLabel_existingLabel (1.61s)
=== RUN   TestAccGithubIssueLabel_importBasic
--- PASS: TestAccGithubIssueLabel_importBasic (1.37s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/github	5.313s

make testacc TEST=./builtin/providers/github/ TESTARGS='-run=TestAccGithubOrganizationWebhook_'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/05/19 15:13:50 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/github/ -v -run=TestAccGithubOrganizationWebhook_ -timeout 120m
=== RUN   TestAccGithubOrganizationWebhook_basic
--- PASS: TestAccGithubOrganizationWebhook_basic (0.87s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/github	0.878s

make testacc TEST=./builtin/providers/github/ TESTARGS='-run=TestAccGithubRepository_'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/05/19 15:16:26 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/github/ -v -run=TestAccGithubRepository_ -timeout 120m
=== RUN   TestAccGithubRepository_basic
--- PASS: TestAccGithubRepository_basic (1.67s)
=== RUN   TestAccGithubRepository_importBasic
--- PASS: TestAccGithubRepository_importBasic (1.12s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/github	2.798s

make testacc TEST=./builtin/providers/github/ TESTARGS='-run=TestAccGithubRepositoryWebhook_'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/05/19 15:34:01 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/github/ -v -run=TestAccGithubRepositoryWebhook_ -timeout 120m
=== RUN   TestAccGithubRepositoryWebhook_basic
--- PASS: TestAccGithubRepositoryWebhook_basic (2.61s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/github	2.614s

[grubernaut]

Nice fix, LGTM

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

If you have found a problem that seems similar to this, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.