Check negative width before blitAntiRect

Since we decrease the width when clipped, it might be negative.

Bug: skia:7858
Change-Id: I348bd07d917905a55bec1a4b4e0915c6528fe6b1
Reviewed-on: https://skia-review.googlesource.com/123927
Auto-Submit: Yuqian Li <liyuqian@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Yuqian Li <liyuqian@google.com>

src/core/SkBlitter.cpp

@@ -147,7 +147,9 @@ void SkBlitter::blitCoverageDeltas(SkCoverageDeltaList* deltas, const SkIRect& c
                 height -= botOverClip;
             }
 
-            this->blitAntiRect(rectX, rectY, width, height, leftAlpha, rightAlpha);
+            if (width >= 0) {
+                this->blitAntiRect(rectX, rectY, width, height, leftAlpha, rightAlpha);
+            }
             y += antiRect.fHeight - 1; // -1 because ++y in the for loop
             continue;
         }