mx_mysql: mx_mysql_statement_close*: Allow NULL

Allow mx_mysql_statement_close* functions to be called with a pointer to
a NULL value. This makes the daemon more robust when the functions are
used as cleanup functions, e.g.

    __attribute__((cleanup(mx_mysql_statement_close)))
    struct mx_mysql_stmt *stmt = mx_mysql_statement_prepare(mysql,"...");
    if (!stmt) {
        fprintf(stderr, "mx_mysql_stmt_prepare: %s\n", mx_mysql_error());
        return;
    }

mx_mysql.c

@@ -1154,8 +1154,8 @@ struct mx_mysql_stmt *mx_mysql_statement_prepare(struct mx_mysql *mysql, char *s
 
 int mx_mysql_statement_close(struct mx_mysql_stmt **stmt)
 {
-    assert(stmt);
-    assert(*stmt);
+    if (*stmt == NULL)
+        return 0;
 
     mx__mysql_stmt_free_result(*stmt);
     mx__mysql_stmt_close(*stmt);
@@ -1169,8 +1169,8 @@ int mx_mysql_statement_close(struct mx_mysql_stmt **stmt)
 
 int mx_mysql_statement_close_no_bind_cleanup(struct mx_mysql_stmt **stmt)
 {
-    assert(stmt);
-    assert(*stmt);
+    if (*stmt == NULL)
+        return 0;
 
     mx__mysql_stmt_free_result(*stmt);
     mx__mysql_stmt_close(*stmt);