Fully initialize array of previous edges in GrGLConvexPolyEffect to please MSAN

BUG=skia:6416

Change-Id: I99c29cd8cb90844e50ed288ba57e28774bf2444c
Reviewed-on: https://skia-review.googlesource.com/10183
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
diff --git a/src/gpu/effects/GrConvexPolyEffect.cpp b/src/gpu/effects/GrConvexPolyEffect.cpp
index d07d591..d94d94a 100644
--- a/src/gpu/effects/GrConvexPolyEffect.cpp
+++ b/src/gpu/effects/GrConvexPolyEffect.cpp
@@ -166,7 +166,9 @@
 class GrGLConvexPolyEffect : public GrGLSLFragmentProcessor {
 public:
     GrGLConvexPolyEffect() {
-        fPrevEdges[0] = SK_ScalarNaN;
+        for (size_t i = 0; i < SK_ARRAY_COUNT(fPrevEdges); ++i) {
+            fPrevEdges[i] = SK_ScalarNaN;
+        }
     }
 
     void emitCode(EmitArgs&) override;