Avoid a segfault when python code fails

PyRun_StringFlags: Returns the result of executing
the code as a Python object, or NULL if an exception was raised.

Author: elpaso

src/python/qgspythonutilsimpl.cpp

@@ -300,7 +300,10 @@ bool QgsPythonUtilsImpl::runStringUnsafe( const QString& command, bool single )
   // (non-unicode strings can be mangled)
   PyObject* obj = PyRun_String( command.toUtf8().data(), single ? Py_single_input : Py_file_input, mMainDict, mMainDict );
   bool res = nullptr == PyErr_Occurred();
-  Py_DECREF( obj );
+  if ( obj )
+  {
+    Py_DECREF( obj );
+  }
 
   // we are done calling python API, release global interpreter lock
   PyGILState_Release( gstate );