client: assign proto to NULL, fix protential coredump
fix multiple building issues under Microsoft Visual C++ 2013.
fix error of comment which misleads users
fix issue #61

This issue was introduced by commit 58816d
Merge pull request #62 from liuyanghejerry/master

fix multiple building issues under Microsoft Visual C++ 2013.
解决冲突。
merge from origin

主要是修复反复重连会崩溃的bug