Don't pass null to memcmp.

This will allow us to add nonnull-attribute to the UBSAN bot.

We are in fact hitting a case where one of the arguments is null and the other
not, which seems dicey.  I think the scenario is comparing the empty pathref
with another path ref that's just been COWed, without any verbs or points yet.

BUG=skia:

Review URL: https://codereview.chromium.org/732643002
2 files changed