Proper error handling in handleAuthResult

[MrBTTF]

Issue description

If block is nil, in line 375 ParsePKIXPublicKey will panic. It happened to me when I was trying to connect GCP sql database and it responded with Access denied for user 'user'@'cloudsqlproxy~xx.xx.xx.x' (using password: YES)

mysql/auth.go

Lines 374 to 375 in fe2230a

	block, _ := pem.Decode(data[1:])
	pkix, err := x509.ParsePKIXPublicKey(block.Bytes)

Error log

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0x6a44f2]

goroutine 1 [running]:
github.com/go-sql-driver/mysql.(*mysqlConn).handleAuthResult(0xc0000a79e0, 0xc0004c8be8, 0x14, 0x14, 0xc0004c8bd0, 0x15, 0x0, 0x0)
	/root/go/pkg/mod/github.com/go-sql-driver/mysql@v1.5.0/auth.go:380 +0x732
github.com/go-sql-driver/mysql.(*connector).Connect(0xc0000103e8, 0xb69bf0, 0xc0000240d8, 0x0, 0x0, 0x0, 0x0)
	/root/go/pkg/mod/github.com/go-sql-driver/mysql@v1.5.0/connector.go:109 +0x596
database/sql.(*DB).conn(0xc000110ea0, 0xb69bf0, 0xc0000240d8, 0xc00025fb01, 0x4e34d7, 0xc0000811e0, 0xc00008e500)
	/usr/local/go/src/database/sql/sql.go:1301 +0x184
database/sql.(*DB).PingContext(0xc000110ea0, 0xb69bf0, 0xc0000240d8, 0x1, 0x1)
	/usr/local/go/src/database/sql/sql.go:799 +0x90
database/sql.(*DB).Ping(...)
	/usr/local/go/src/database/sql/sql.go:817
github.com/GoogleCloudPlatform/cloudsql-proxy/proxy/dialers/mysql.DialCfg(0xc00025fd68, 0xc00025fd68, 0x52632e, 0xc0002acbd0)
	/root/go/pkg/mod/github.com/!google!cloud!platform/cloudsql-proxy@v1.19.1/proxy/dialers/mysql/hook.go:93 +0x1c5
github.com/qreuz/common-go/pkg/utils.InitDB(0xc0000f78c0, 0xaaf751, 0x9, 0xc0000d8600, 0x0, 0xc00025feb0, 0x52a3a5)
...

Suggestion

I suggest to handle it in this way

block, rest := pem.Decode(data[1:])
if block == nil {
	return fmt.Errorf("No Pem data found, data: %s", rest)
}

Configuration

Driver version (or git SHA): v1.5.0

Go version: go version go1.16 linux/amd64

Server version: MySQL 8