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

Source/core/rendering/RenderMultiColumnFlowThread.cpp

Index: Source/core/rendering/RenderMultiColumnFlowThread.cpp
diff --git a/Source/core/rendering/RenderMultiColumnFlowThread.cpp b/Source/core/rendering/RenderMultiColumnFlowThread.cpp
index e93c68aedd5e3c88be5e37d863e34ea204b25732..a26ce28cb0a26d71238095bcbbe44db3afa35ed4 100644
--- a/Source/core/rendering/RenderMultiColumnFlowThread.cpp
+++ b/Source/core/rendering/RenderMultiColumnFlowThread.cpp
@@ -271,6 +271,13 @@ LayoutUnit RenderMultiColumnFlowThread::initialLogicalWidth() const
 
 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);
 }