[Backport] Fix a possible invalid memory access

This could happen if the signal target was removed in the callback, which could lead to an invalid signalId being returned. I guess this could lead to the signal receiver destroyed out of turn.

(cherry picked from commit MeVisLab/pythonqt@b5a88e1)

Author: usiems

src/PythonQtSignalReceiver.cpp

@@ -270,9 +270,9 @@ int PythonQtSignalReceiver::qt_metacall(QMetaObject::Call c, int id, void **argu
   bool shouldDelete = false;
   for(const PythonQtSignalTarget& t : _targets) {
     if (t.slotId() == id) {
+      const int sigId = t.signalId();
       t.call(arguments);
       // if the signal is the last destroyed signal, we delete ourselves
-      int sigId = t.signalId();
       if ((sigId == _destroyedSignal1Id) || (sigId == _destroyedSignal2Id)) {
         _destroyedSignalCount--;
         if (_destroyedSignalCount == 0) {