SkSpan cleanup

* Check index in operator[]
* Use begin/end in const versions
* Remove returning SkSpan{} when empty for first, last, and subspan.

Change-Id: I0348ff8effb01e9175c5afaa52ddb0724f86218d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/303259
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Herb Derby <herb@google.com>
1 file changed