mx_mxql: Initialize s->func

When we run into a (very unlikely) early error condition, the error
macros might output s->func. While glibc printf functions output
"(null)" if NULL is passed to "%s", this is not guaranteed.

For example, gcc transforms `printf("%s\n", ptr)` into `puts(ptr)`.

mx_mysql.c

@@ -962,6 +962,7 @@ static int mx_mysql_statement_init(struct mx_mysql *mysql, struct mx_mysql_stmt
     s = mx_calloc_forever(1, sizeof(*s));
 
     s->mysql       = mysql;
+    s->func = "";
 
     do {
         res = mx__mysql_stmt_init(s);