Fix UB in SkString::equal(nullptr) when empty.
Fix UB in SkString::equal(nullptr) when empty.

It is undefined behavior to memcmp with a nullptr. It is possible to
get into such a state if the SkString is itself empty and compared
against nullptr using SkString::equal.

Change-Id: I7a55d7beb5ed454c73a56f47c3729f3790a5fd78
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/306951
Auto-Submit: Ben Wagner <53965154ffeecc4a10886eab9259e95965f2979a@google.com>
Commit-Queue: Ben Wagner <53965154ffeecc4a10886eab9259e95965f2979a@google.com>
Commit-Queue: Mike Klein <14574f09dfa9b4e14759b88c3426a495a0e627b0@google.com>
Reviewed-by: Mike Klein <14574f09dfa9b4e14759b88c3426a495a0e627b0@google.com>

aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia