lavf/mux: Fix assignments in if()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 1ac5a8d7e3)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mux.c

@@ -395,7 +395,7 @@ int avformat_write_header(AVFormatContext *s, AVDictionary **options)
             return ret;
     }
 
-    if ((ret = init_pts(s) < 0))
+    if ((ret = init_pts(s)) < 0)
         return ret;
 
     return 0;