Make Regexp::Simplify() return a null pointer when stopped early.

Change-Id: I19e447e6c7dec34201299a12104e72bf7e792a9e
Reviewed-on: https://code-review.googlesource.com/c/re2/+/57570
Reviewed-by: Paul Wankadia <junyer@google.com>
(cherry picked from commit ca11026a032ce2a3de4b3c389ee53d2bdc8794d6)
Reviewed-on: https://code-review.googlesource.com/c/re2/+/57571
2 files changed