MXS-852: Fix prepared statement collection

If a prepared statement response was collected into one buffer, it doesn't
need to be processed again. By jumping directly to the routing of the
collected result, we prevent the unnecessary splitting of buffers that
appears to happend with continuous preparations of prepared statements.

server/core/modutil.cc

@@ -1310,7 +1310,7 @@ bool modutil_ignorable_ping(DCB *dcb)
     return rval;
 }
 
-const char format_str[] = "COM_UNKNOWN(%02x)";
+const char format_str[] = "COM_UNKNOWN(%02hhx)";
 
 // The message always fits inside the buffer
 thread_local char unknow_type[sizeof(format_str)] = "";