[New Multicolumn] Guard against zero or negative space shortage.

Source/core/rendering/RenderMultiColumnFlowThread.cpp

 /*
  * Copyright (C) 2012 Apple Inc.  All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 253 matching lines @@
     computedValues.m_position = logicalTop;
 }
 
 LayoutUnit RenderMultiColumnFlowThread::initialLogicalWidth() const
 {
     return columnWidth();
 }
 
 void RenderMultiColumnFlowThread::setPageBreak(LayoutUnit offset, LayoutUnit spaceShortage)
 {
+    // Only positive values are interesting (and allowed) here. Zero space shortage may be reported
+    // when we're at the top of a column and the element has zero height. Ignore this, and also
+    // ignore any negative values, which may occur when we set an early break in order to honor
+    // widows in the next column.
+    if (spaceShortage <= 0)
+        return;
+
     if (RenderMultiColumnSet* multicolSet = toRenderMultiColumnSet(regionAtBlockOffset(offset)))
         multicolSet->recordSpaceShortage(spaceShortage);
 }
 
 void RenderMultiColumnFlowThread::updateMinimumPageHeight(LayoutUnit offset, LayoutUnit minHeight)
 {
     if (RenderMultiColumnSet* multicolSet = toRenderMultiColumnSet(regionAtBlockOffset(offset)))
         multicolSet->updateMinimumColumnHeight(minHeight);
 }
 
@@ skipping 15 matching lines @@
 }
 
 bool RenderMultiColumnFlowThread::isPageLogicalHeightKnown() const
 {
     if (RenderMultiColumnSet* columnSet = lastMultiColumnSet())
         return columnSet->computedColumnHeight();
     return false;
 }
 
 }