Reland "Fixed DSL assertion error on source files containing nulls"

This is a reland of db38ad7b142e44aa4718d7c71d47f680d247cff0

Original change's description:
> 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>

Bug: oss-fuzz:38107
Change-Id: Idb1a6b7c64d2bb954edadae828d6de808158fd3f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/448660
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
4 files changed