Don't reset match_[0]. It isn't necessary.

Change-Id: I0a107ebaa00844e245fce632e370e47cd50d34cc
Reviewed-on: https://code-review.googlesource.com/4641
Reviewed-by: Paul Wankadia <junyer@google.com>

re2/nfa.cc

diff --git a/re2/nfa.cc b/re2/nfa.cc
index 94c65a6..16bbe31 100644
--- a/re2/nfa.cc
+++ b/re2/nfa.cc
@@ -580,11 +580,8 @@
         flag = Prog::EmptyFlags(context, p);
       }
 
-      // Steal match storage (cleared but unused as of yet)
-      // temporarily to hold match boundaries for new thread.
       match_[0] = p;
       AddToThreadq(runq, start_, flag, p, match_);
-      match_[0] = NULL;
     }
 
     // If all the threads have died, stop early.