Fix assert in SkShadowVertices constructor

Change-Id: Icedbe70079d41234c98330af64d71a4e60b67f1c
Reviewed-on: https://skia-review.googlesource.com/7796
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
1 file changed