Aiieee. Add a missing underscore.

Change-Id: I38ebf3a6886bf8142836565652110f343a56877a
Reviewed-on: https://code-review.googlesource.com/c/re2/+/53872
Reviewed-by: Paul Wankadia <junyer@google.com>
diff --git a/re2/set.cc b/re2/set.cc
index 6c377aa..87db6b7 100644
--- a/re2/set.cc
+++ b/re2/set.cc
@@ -125,9 +125,9 @@
   if (dfa_failed) {
     if (options_.log_errors())
       LOG(ERROR) << "DFA out of memory: "
-                 << "program size " << prog->size() << ", "
-                 << "list count " << prog->list_count() << ", "
-                 << "bytemap range " << prog->bytemap_range();
+                 << "program size " << prog_->size() << ", "
+                 << "list count " << prog_->list_count() << ", "
+                 << "bytemap range " << prog_->bytemap_range();
     if (error_info != NULL)
       error_info->kind = kOutOfMemory;
     return false;