Handle nullptr from asTextureRef

This can fail, if the special image dimensions are larger than the maximum
allowable render target dimensions.

BUG=chromium:666519
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2520043003

Review-Url: https://codereview.chromium.org/2520043003

src/image/SkImage.cpp

@@ -352,6 +352,9 @@ sk_sp<SkImage> SkImage::makeWithFilter(const SkImageFilter* filter, const SkIRec
     if (result->isTextureBacked()) {
         GrContext* context = result->getContext();
         sk_sp<GrTexture> texture = result->asTextureRef(context);
+        if (!texture) {
+            return nullptr;
+        }
         fullSize = SkIRect::MakeWH(texture->width(), texture->height());
     }
 #endif