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>
1 file changed