Fix a latent bug in Regexp::Walker<T>::Reset().

Change-Id: I0e2732fa6a0e2d73f20e1dc872720796cc1c22be
Reviewed-on: https://code-review.googlesource.com/c/re2/+/48110
Reviewed-by: Paul Wankadia <junyer@google.com>
(cherry picked from commit af3455996c0213fa030546eeba0cc44b947b1de8)
Reviewed-on: https://code-review.googlesource.com/c/re2/+/48111
1 file changed