etcdmain: check error before assigning peer transport

[yichengq]

Or it may panic when new transport fails, e.g., TLS info is invalid.

example when providing nonexistant TLS info:

22:06:00 proxy | panic: runtime error: invalid memory address or nil pointer dereference
22:06:00 proxy | [signal 0xb code=0x1 addr=
22:06:00 proxy | 0x80 pc=0x660bf]
22:06:00 proxy | goroutine 1 [running]:
22:06:00 proxy | github.com/coreos/etcd/etcdmain.startProxy(0x820b1a900, 0x0, 0x0)
22:06:00 proxy |    /Users/unihorn/gopath/src/github.com/coreos/etcd/gopath/src/github.com/coreos/etcd/etcdmain/etcd.go:317 +0x22f

[xiang90]

LGTM