Fix some Coverity dereference null pointer warnings

Author: nyalldawson

src/core/symbology-ng/qgsfillsymbollayerv2.cpp

@@ -3455,7 +3455,7 @@ void QgsCentroidFillSymbolLayerV2::renderPolygon( const QPolygonF& points, QList
         if ( context.geometryPartCount() > 1 )
         {
           const QgsGeometry *geom = feature->constGeometry();
-          const QgsGeometryCollectionV2* geomCollection = dynamic_cast<const QgsGeometryCollectionV2*>( geom->geometry() );
+          const QgsGeometryCollectionV2* geomCollection = static_cast<const QgsGeometryCollectionV2*>( geom->geometry() );
 
           double area = 0;
           double areaBiggest = 0;

src/gui/qgsattributeformeditorwidget.cpp

@@ -66,7 +66,7 @@ void QgsAttributeFormEditorWidget::setMode( QgsAttributeFormEditorWidget::Mode m
 
 void QgsAttributeFormEditorWidget::setIsMixed( bool mixed )
 {
-  if ( mixed )
+  if ( mWidget && mixed )
     mWidget->showIndeterminateState( );
   mMultiEditButton->setIsMixed( mixed );
   mIsMixed = mixed;
@@ -123,7 +123,8 @@ void QgsAttributeFormEditorWidget::resetValue()
 {
   mIsChanged = false;
   mBlockValueUpdate = true;
-  mWidget->setValue( mPreviousValue );
+  if ( mWidget )
+    mWidget->setValue( mPreviousValue );
   mBlockValueUpdate = false;
 
   switch ( mMode )