initialize val in SkTHash Slot

Found with a GCC compiler warning.
Seems like a sane idea to keep Slots fully initialized.

Change-Id: I7a7bd4ccc4a6800d6e262aa7c616d3cab36d74dc
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/227121
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
1 file changed