avformat/srtdec: Fix pointer corruption

This fixes use of uninitialized memory and possible out of array access

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/srtdec.c

@@ -63,10 +63,12 @@ static int64_t get_pts(const char **buf, int *duration,
             int64_t start = (hh1*3600LL + mm1*60LL + ss1) * 1000LL + ms1;
             int64_t end   = (hh2*3600LL + mm2*60LL + ss2) * 1000LL + ms2;
             *duration = end - start;
-            *buf += strcspn(*buf, "\n") + 1;
+            *buf += strcspn(*buf, "\n");
+            *buf += !!**buf;
             return start;
         }
-        *buf += strcspn(*buf, "\n") + 1;
+        *buf += strcspn(*buf, "\n");
+        *buf += !!**buf;
     }
     return AV_NOPTS_VALUE;
 }