fftools/ffmpeg: prefer real errors over EOF in err_merge()

Fixes an issue in 6.1 when reading a corrupted file with -xerror would
exit with success. This specific issue is not present in master, but
this should generally be a more robust behaviour.

Reported-by: Andrej Peterka
(cherry picked from commit d1fa39d08d)
Signed-off-by: Anton Khirnov <anton@khirnov.net>

fftools/ffmpeg.h

@@ -883,11 +883,12 @@ void update_benchmark(const char *fmt, ...);
 /**
  * Merge two return codes - return one of the error codes if at least one of
  * them was negative, 0 otherwise.
- * Currently just picks the first one, eventually we might want to do something
- * more sophisticated, like sorting them by priority.
  */
 static inline int err_merge(int err0, int err1)
 {
+    // prefer "real" errors over EOF
+    if ((err0 >= 0 || err0 == AVERROR_EOF) && err1 < 0)
+        return err1;
     return (err0 < 0) ? err0 : FFMIN(err1, 0);
 }