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>
diff --git a/re2/walker-inl.h b/re2/walker-inl.h
index 032b8ac..310be54 100644
--- a/re2/walker-inl.h
+++ b/re2/walker-inl.h
@@ -150,7 +150,7 @@
   if (stack_ && stack_->size() > 0) {
     LOG(DFATAL) << "Stack not empty.";
     while (stack_->size() > 0) {
-      delete stack_->top().child_args;
+      delete[] stack_->top().child_args;
       stack_->pop();
     }
   }