fixed use-after-free in sksl switch statements

Bug: skia:
Change-Id: I66ef1cd2af9c654bfa40a71b2218cfde49f3a54e
Reviewed-on: https://skia-review.googlesource.com/24329
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
3 files changed