Always mark features as valid when added to memory provider

For other providers features will automatically be made valid
through the proces of adding to the provider's storage itself
and then later retrieving via iterators. But the memory
provider uses a direct copy of the feature, so if we don't
explicitly mark features as valid the provider may be
returning features incorrectly marked as invalid.

(cherry-picked from f2b70cf)

Author: nyalldawson

src/providers/memory/qgsmemoryprovider.cpp

@@ -334,6 +334,7 @@ bool QgsMemoryProvider::addFeatures( QgsFeatureList & flist )
     mFeatures[mNextFeatureId] = *it;
     QgsFeature& newfeat = mFeatures[mNextFeatureId];
     newfeat.setFeatureId( mNextFeatureId );
+    newfeat.setValid( true );
     it->setFeatureId( mNextFeatureId );
 
     // update spatial index