Only access the scissorState's rect when it is valid

It is odd asan/valgrind hasn't been complaining about this

Change-Id: I8e7952e6fc554450ff8c5c5940500f09be48d6b0
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/255144
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
5 files changed