_mssql.pyx: Zero init global last_msg_* vars

Otherwise they can have random garbage in them.

 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
Merge pull request #152 from pymssql/clear_zero_init_global_mssql_last_msg_stuff

_mssql.pyx: Zero init global last_msg_* vars