Fixed SkSL use-after-free fuzzer bug and added defensive code to catch such problems in the future.

Bug: skia:7558
Change-Id: I5098c0ed08f2328828969e819db7785270b26656
Reviewed-on: https://skia-review.googlesource.com/111460
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
6 files changed