Address a `-Wunused-but-set-variable' warning from Clang 13.x.
Address a `-Wunused-but-set-variable' warning from Clang 13.x.

Change-Id: Ib7c4efdd9eba4987c080d4e78a5bb42771c399e0
Reviewed-on: https://code-review.googlesource.com/c/re2/+/59630
Reviewed-by: Paul Wankadia <62d4e38bb1380f055ec4b899baa75e52af09c5f7@google.com>

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