Assert that only complete COM_STMT_PREPARE responses are returned

Asserting that only a complete COM_STMT_PREPARE is returned when the
prepared statement preparation is extracted will guarantee that the
protocol works as expected.

server/modules/protocol/MySQL/mysql_common.c

@@ -1661,6 +1661,23 @@ bool mxs_mysql_extract_ps_response(GWBUF* buffer, MXS_PS_RESPONSE* out)
         out->parameters = gw_mysql_get_byte2(params);
         out->warnings = gw_mysql_get_byte2(warnings);
         rval = true;
+
+#ifdef SS_DEBUG
+        // Make sure that the PS response contains the whole response
+        bool more;
+        modutil_state state;
+        int n_eof = modutil_count_signal_packets(buffer, 0, &more, &state);
+        int n_expected = 0;
+        if (out->columns)
+        {
+            n_expected++;
+        }
+        if (out->parameters)
+        {
+            n_expected++;
+        }
+        ss_dassert(n_eof == n_expected);
+#endif
     }
 
     return rval;