Fixed DSL assertion error on source files containing nulls

The assertion was there to make sure we weren't running off the end of
the source, but naturally fails in the presence of legitimate embedded
nulls.

Change-Id: I3b80499e9b182c9ea046c479f35d7a965d548401
Bug: oss-fuzz:38107
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/447182
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
4 files changed