Avoid assertion in MSAA rendering buffer about indexed draw with zero index count

Also add some GrMesh::validate() calls earlier to make future debugging easier. Previously this only asserted when the draws executed, now it asserts as soon as the draw is prepared in the op subclass.

Bug: skia:
Change-Id: Ibdd4488b9a9263f0ed337a2c8c6066bc36297001
Reviewed-on: https://skia-review.googlesource.com/16029
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
diff --git a/src/gpu/ops/GrMSAAPathRenderer.cpp b/src/gpu/ops/GrMSAAPathRenderer.cpp
index b2782df..49f8259 100644
--- a/src/gpu/ops/GrMSAAPathRenderer.cpp
+++ b/src/gpu/ops/GrMSAAPathRenderer.cpp
@@ -380,11 +380,9 @@
             quads.indices = nullptr;
             quads.nextIndex = nullptr;
         }
-
         // fill buffers
         for (int i = 0; i < fPaths.count(); i++) {
             const PathInfo& pathInfo = fPaths[i];
-
             if (!this->createGeom(lines,
                                   quads,
                                   pathInfo.fPath,
@@ -423,7 +421,12 @@
             lineMeshes.fVertexCount = lineVertexOffset;
             lineMeshes.fBaseVertex = firstLineVertex;
 
-            target->draw(lineGP.get(), this->pipeline(), lineMeshes);
+            // We can get line vertices from path moveTos with no actual segments and thus no index
+            // count. We assert that indexed draws contain a positive index count, so bail here in
+            // that case.
+            if (!fIsIndexed || lineIndexOffset) {
+                target->draw(lineGP.get(), this->pipeline(), lineMeshes);
+            }
         }
 
         if (quadVertexOffset) {