[processing] Fix potential crash when calling processing.run with

custom context, but no feedback object

(cherry-picked from 977e14b)

Author: nyalldawson

python/plugins/processing/core/Processing.py

@@ -135,6 +135,9 @@ def runAlgorithm(algOrName, parameters, onFinish=None, feedback=None, context=No
         if context is None:
             context = dataobjects.createContext(feedback)
 
+        if context.feedback() is None:
+            context.setFeedback(feedback)
+
         ok, msg = alg.checkParameterValues(parameters, context)
         if not ok:
             msg = Processing.tr('Unable to execute algorithm\n{0}').format(msg)