Make DidReflowChildren() deal with a null first frame. b=372483 r+sr=rbs

layout/mathml/base/src/nsMathMLContainerFrame.cpp

@@ -1417,7 +1417,8 @@ void
 nsMathMLContainerFrame::DidReflowChildren(nsIFrame* aFirst, nsIFrame* aStop)
 
 {
-  NS_PRECONDITION(aFirst, "expected a frame");
+  if (NS_UNLIKELY(!aFirst))
+    return;
 
   for (nsIFrame* frame = aFirst;
        frame != aStop;

layout/mathml/base/src/nsMathMLContainerFrame.h

@@ -311,6 +311,7 @@ protected:
    * Call DidReflow() if the NS_FRAME_IN_REFLOW frame bit is set on aFirst and
    * all its next siblings up to, but not including, aStop.
    * aStop == nsnull meaning all next siblings with the bit set.
+   * The method does nothing if aFirst == nsnull.
    */
   void DidReflowChildren(nsIFrame* aFirst, nsIFrame* aStop = nsnull);
 };