handle null vertex or index buffers in batch

BUG=skia:

Review URL: https://codereview.chromium.org/979343002
diff --git a/src/gpu/GrTessellatingPathRenderer.cpp b/src/gpu/GrTessellatingPathRenderer.cpp
index 955e6c1..1170e1c 100644
--- a/src/gpu/GrTessellatingPathRenderer.cpp
+++ b/src/gpu/GrTessellatingPathRenderer.cpp
@@ -1515,6 +1515,12 @@
                                                               count,
                                                               &vertexBuffer,
                                                               &firstVertex);
+
+        if (!vertices) {
+            SkDebugf("Could not allocate vertices\n");
+            return;
+        }
+
         LOG("emitting %d verts\n", count);
         void* end = polys_to_triangles(polys, fillType, vertices);
         int actualCount = static_cast<int>(