handle null vertex or index buffers in batch

BUG=skia:

Review URL: https://codereview.chromium.org/979343002
diff --git a/src/gpu/GrOvalRenderer.cpp b/src/gpu/GrOvalRenderer.cpp
index fc8c184..20ae77c 100644
--- a/src/gpu/GrOvalRenderer.cpp
+++ b/src/gpu/GrOvalRenderer.cpp
@@ -762,6 +762,11 @@
                                                               &vertexBuffer,
                                                               &firstVertex);
 
+        if (!vertices || !batchTarget->quadIndexBuffer()) {
+            SkDebugf("Could not allocate buffers\n");
+            return;
+        }
+
         CircleVertex* verts = reinterpret_cast<CircleVertex*>(vertices);
 
         for (int i = 0; i < instanceCount; i++) {
@@ -1012,6 +1017,11 @@
                                                               &vertexBuffer,
                                                               &firstVertex);
 
+        if (!vertices || !batchTarget->quadIndexBuffer()) {
+            SkDebugf("Could not allocate buffers\n");
+            return;
+        }
+
         EllipseVertex* verts = reinterpret_cast<EllipseVertex*>(vertices);
 
         for (int i = 0; i < instanceCount; i++) {
@@ -1303,6 +1313,11 @@
                                                               &vertexBuffer,
                                                               &firstVertex);
 
+        if (!vertices || !batchTarget->quadIndexBuffer()) {
+            SkDebugf("Could not allocate buffers\n");
+            return;
+        }
+
         DIEllipseVertex* verts = reinterpret_cast<DIEllipseVertex*>(vertices);
 
         for (int i = 0; i < instanceCount; i++) {
@@ -1692,6 +1707,11 @@
                                                               &vertexBuffer,
                                                               &firstVertex);
 
+        if (!vertices) {
+            SkDebugf("Could not allocate vertices\n");
+            return;
+        }
+
         CircleVertex* verts = reinterpret_cast<CircleVertex*>(vertices);
 
         for (int i = 0; i < instanceCount; i++) {
@@ -1896,6 +1916,11 @@
                                                               &vertexBuffer,
                                                               &firstVertex);
 
+        if (!vertices) {
+            SkDebugf("Could not allocate vertices\n");
+            return;
+        }
+
         EllipseVertex* verts = reinterpret_cast<EllipseVertex*>(vertices);
 
         for (int i = 0; i < instanceCount; i++) {