Fix bug with "%%" in error format

* src/doprnt.c (doprnt): Format "%%" correctly.
Problem reported by Philipp Stephani in:
http://lists.gnu.org/archive/html/emacs-devel/2017-05/msg00901.html

1 files changed, 3 insertions, 1 deletions

diff --git a/src/doprnt.c b/src/doprnt.c
index bed9350f4a6..418601acb02 100644
--- a/src/doprnt.c
+++ b/src/doprnt.c
@@ -438,7 +438,9 @@ doprnt (char *buffer, ptrdiff_t bufsize, const char *format,
               }
 
             case '%':
-              fmt--;    /* Drop thru and this % will be treated as normal */
+              /* Treat this '%' as normal.  */
+              fmt0 = fmt - 1;
+              break;
             }
         }