[labeling] Fix incorrect label placement after moving an 'unplaced' l…

…abel

For unplaced labels, we can't calculate the new position relative to
the original stored label placement, because we're not actually
showing the 'unplaced' label at the stored position...

(cherry picked from commit 7e6b185)

src/app/qgsmaptoollabel.cpp

@@ -510,11 +510,19 @@ bool QgsMapToolLabel::currentLabelDataDefinedPosition( double &x, bool &xSuccess
     return false;
   }
 
-  QgsAttributes attributes = f.attributes();
-  if ( !attributes.at( xCol ).isNull() )
-    x = attributes.at( xCol ).toDouble( &xSuccess );
-  if ( !attributes.at( yCol ).isNull() )
-    y = attributes.at( yCol ).toDouble( &ySuccess );
+  if ( mCurrentLabel.pos.isUnplaced )
+  {
+    xSuccess = false;
+    ySuccess = false;
+  }
+  else
+  {
+    QgsAttributes attributes = f.attributes();
+    if ( !attributes.at( xCol ).isNull() )
+      x = attributes.at( xCol ).toDouble( &xSuccess );
+    if ( !attributes.at( yCol ).isNull() )
+      y = attributes.at( yCol ).toDouble( &ySuccess );
+  }
 
   return true;
 }