Canvas: fix crash in BaseRenderingContext2D::getImageData

The code prior to this patch did not fail if GetImage returns nullptr.

Bug: 1149954
Change-Id: If87d16ea2573cc8e5db1e419b743d37bc5062ceb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2545035Reviewed-by: Yi Xu <yiyix@chromium.org>
Commit-Queue: ccameron <ccameron@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828940}

third_party/blink/renderer/modules/canvas/canvas2d/base_rendering_context_2d.cc

@@ -1746,7 +1746,8 @@ ImageData* BaseRenderingContext2D::getImageData(
   }
 
   // Read pixels into |contents|.
-  bool read_pixels_successful =
+  if (snapshot) {
+    const bool read_pixels_successful =
         snapshot->PaintImageForCurrentFrame().readPixels(
             image_info, contents.Data(), image_info.minRowBytes(), sx, sy);
     if (!read_pixels_successful) {
@@ -1754,6 +1755,7 @@ ImageData* BaseRenderingContext2D::getImageData(
           snapshot->PaintImageForCurrentFrame().GetSkImageInfo().bounds();
       DCHECK(!bounds.intersect(SkIRect::MakeXYWH(sx, sy, sw, sh)));
     }
+  }
 
   // Wrap |contents| in an ImageData.
   DOMArrayBuffer* array_buffer = DOMArrayBuffer::Create(std::move(contents));