_mssql.pyx: Zero init global last_msg_* vars

Otherwise they can have random garbage in them. I discovered this while testing PR #151 on Ubuntu 13, because that change causes a connection failure to no longer clobber `_mssql_last_msg_str`, so that you can then see the random garbage in it.

```cython
 cdef char *_mssql_last_msg_str = <char *>PyMem_Malloc(PYMSSQL_MSGSIZE)
+_mssql_last_msg_str[0] = <char>0
 cdef char *_mssql_last_msg_srv = <char *>PyMem_Malloc(PYMSSQL_MSGSIZE)
+_mssql_last_msg_srv[0] = <char>0
 cdef char *_mssql_last_msg_proc = <char *>PyMem_Malloc(PYMSSQL_MSGSIZE)
+_mssql_last_msg_proc[0] = <char>0
```