Fix CVE-2015-1345.  From upstream commit
83a95bd8c8561875b948cadd417c653dbe7ef2e2
by Yuliy Pisetsky <ypisetsky@fb.com>.

diff --git a/src/kwset.c b/src/kwset.c
index 4003c8d..376f7c3 100644
--- a/src/kwset.c
+++ b/src/kwset.c
@@ -643,6 +643,8 @@ bmexec_trans (kwset_t kwset, char const *text, size_t size)
                     if (! tp)
                       return -1;
                     tp++;
+                    if (ep <= tp)
+                      break;
                   }
               }
           }