nutdec: Always return a value from nut_read_timestamp()

The function is a callback that is called by ff_gen_search with
a constant stream index.

Avoid a false positive on older gcc version.
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

libavformat/nutdec.c

@@ -910,7 +910,7 @@ static int64_t nut_read_timestamp(AVFormatContext *s, int stream_index,
     else if (stream_index == -2)
         return back_ptr;
 
-    assert(0);
+    return AV_NOPTS_VALUE;
 }
 
 static int read_seek(AVFormatContext *s, int stream_index,