Remove a pointer chase from Regexp::Walker<>.
Remove a pointer chase from Regexp::Walker<>.

Change-Id: I452034ebc21d087ebd58eecc41d9824bf6fbce0b
Reviewed-on: https://code-review.googlesource.com/c/re2/+/56552
Reviewed-by: Paul Wankadia <62d4e38bb1380f055ec4b899baa75e52af09c5f7@google.com>

google/re2,google/re2,google/re2,google/re2,google/re2