Fix major(?) memory leak with python code execution

(cherry-picked from 73733a6)

src/python/qgspythonutilsimpl.cpp

@@ -298,9 +298,9 @@ bool QgsPythonUtilsImpl::runStringUnsafe( const QString& command, bool single )
   // TODO: convert special characters from unicode strings u"..." to \uXXXX
   // so that they're not mangled to utf-8
   // (non-unicode strings can be mangled)
-  PyRun_String( command.toUtf8().data(), single ? Py_single_input : Py_file_input, mMainDict, mMainDict );
-
+  PyObject* obj = PyRun_String( command.toUtf8().data(), single ? Py_single_input : Py_file_input, mMainDict, mMainDict );
   bool res = nullptr == PyErr_Occurred();
+  Py_DECREF( obj );
 
   // we are done calling python API, release global interpreter lock
   PyGILState_Release( gstate );
@@ -581,6 +581,8 @@ bool QgsPythonUtilsImpl::evalString( const QString& command, QString& result )
   if ( success )
     result = PyObjectToQString( res );
 
+  Py_XDECREF( res );
+
   // we are done calling python API, release global interpreter lock
   PyGILState_Release( gstate );