Add a conn != nil check

[biter777]

go version go1.13 linux/amd64 Ubuntu 18.04.3 LTS

Т.к. в clientHandler() в рутине происходит conn, err = c.Dial(), то бывают такие случаи...

panic: runtime error: invalid memory address or nil pointer dereference [signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x12ee814]

goroutine 39 [running]: github.com/valyala/gorpc.clientHandleConnection(0xc0002d0000, 0x0, 0x0) /mnt/hdd/home/biter/go/src/github.com/valyala/gorpc/client.go:688 +0x2b4 github.com/valyala/gorpc.clientHandler(0xc0002d0000) /mnt/hdd/home/biter/go/src/github.com/valyala/gorpc/client.go:673 +0x4f6 created by github.com/valyala/gorpc.(*Client).Start /mnt/hdd/home/biter/go/src/github.com/valyala/gorpc/client.go:157 +0x241