avformat/subfile: Assert that whence is a known case

This may help CID1452449 Uninitialized scalar variable

Sponsored-by: Sovereign Tech Fund
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 426d8c84c3)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/subfile.c

@@ -137,6 +137,8 @@ static int64_t subfile_seek(URLContext *h, int64_t pos, int whence)
     case SEEK_END:
         new_pos = end + pos;
         break;
+    default:
+        av_assert0(0);
     }
     if (new_pos < c->start)
         return AVERROR(EINVAL);