Bug 1324072 - Use the frame's BStart, not rect's (fixes typo from bug 789096). r=jfkthame

layout/mathml/nsMathMLmtableFrame.cpp

@@ -858,8 +858,9 @@ nsMathMLmtableWrapperFrame::Reflow(nsPresContext*           aPresContext,
                        aReflowInput.ComputedSizeAsContainerIfConstrained());
       blockSize = rect.BSize(wm);
       do {
-        dy += rect.BStart(wm);
-        frame = frame->GetParent();
+        nsIFrame* parent = frame->GetParent();
+        dy += frame->BStart(wm, parent->GetSize());
+        frame = parent;
       } while (frame != this);
     }
   }