Return null if clipshader fails

This allows us to try skvm blitter, which might succeed with
the clipshader.

Change-Id: I771d4d389ae59ffd76e742cd223798f17582fedc
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/383096
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Reed <reed@google.com>

src/core/SkRasterPipelineBlitter.cpp

@@ -172,6 +172,8 @@ SkBlitter* SkRasterPipelineBlitter::Create(const SkPixmap& dst,
             clipP->append(SkRasterPipeline::store_src_a, storage->fA);
             blitter->fClipShaderBuffer = storage->fA;
             is_constant = false;
+        } else {
+            return nullptr;
         }
     }