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

@@ -1746,13 +1746,15 @@ ImageData* BaseRenderingContext2D::getImageData(
   }
 
   // Read pixels into |contents|.
-  bool read_pixels_successful =
-      snapshot->PaintImageForCurrentFrame().readPixels(
-          image_info, contents.Data(), image_info.minRowBytes(), sx, sy);
-  if (!read_pixels_successful) {
-    SkIRect bounds =
-        snapshot->PaintImageForCurrentFrame().GetSkImageInfo().bounds();
-    DCHECK(!bounds.intersect(SkIRect::MakeXYWH(sx, sy, sw, sh)));
+  if (snapshot) {
+    const bool read_pixels_successful =
+        snapshot->PaintImageForCurrentFrame().readPixels(
+            image_info, contents.Data(), image_info.minRowBytes(), sx, sy);
+    if (!read_pixels_successful) {
+      SkIRect bounds =
+          snapshot->PaintImageForCurrentFrame().GetSkImageInfo().bounds();
+      DCHECK(!bounds.intersect(SkIRect::MakeXYWH(sx, sy, sw, sh)));
+    }
   }
 
   // Wrap |contents| in an ImageData.