tests: fix test case panic error
tests: fix test case panic error

Fix test case in tests/e2e/etcd_config_test.go:311, here should
check if it is nil. As the following errors:

--- FAIL: TestGrpcproxyAndCommonName (0.00s)
    etcd_config_test.go:304: Unexpected error: fork/exec ../../bin/etcd: no such file or directory
    etcd_config_test.go:309: Unexpected error: fork/exec ../../bin/etcd: no such file or directory
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
        panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x56bd96]

coreos/etcd,ajityagaty/etcd,ajityagaty/etcd,mqliang/etcd,gyuho/etcd,etcd-io/etcd,mkumatag/etcd,mkumatag/etcd,rnaveiras/etcd,mqliang/etcd,etcd-io/etcd,mkumatag/etcd,rnaveiras/etcd,etcd-io/etcd,mqliang/etcd,rnaveiras/etcd,coreos/etcd,ajityagaty/etcd,gyuho/etcd