client: fix nil dereference when closing conns

[youjiali1995]

Signed-off-by: youjiali1995 [email protected]

Introduced by #464. Connections can be nil if fails to connect some stores.

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x10 pc=0x1a39f6b]

goroutine 47779631 [running]:
google.golang.org/grpc.(*ClientConn).Close(0x0)
	/go/pkg/mod/google.golang.org/[email protected]/clientconn.go:1059 +0x4b
github.com/tikv/client-go/v2/internal/client.(*connArray).Close(0xc326b88140)
	/go/pkg/mod/github.com/tikv/client-go/[email protected]/internal/client/client.go:242 +0x56
github.com/tikv/client-go/v2/internal/client.(*connArray).Init(0xc326b88140, {0xc001cd8978, 0x12}, {{0x0, 0x0}, {0x0, 0x0}, {0x0, 0x0}, {0x0, ...}}, ...)
	/go/pkg/mod/github.com/tikv/client-go/[email protected]/internal/client/client.go:202 +0xee6

[sticnarf]

In which case may c be nil? Does it mean DialContext may return nil, nil?

[youjiali1995]

In which case may c be nil? Does it mean DialContext may return nil, nil?

No. As long as DialContext returns error, c will be nil.

[sticnarf]

In which case may c be nil? Does it mean DialContext may return nil, nil?

No. As long as DialContext returns error, c will be nil.

I misread the code. I see.