avformat/wtvdec: Check pointer before use

Fixes out of array read
Fixes: 049fdf78565f1ce5665df236d90f8657/asan_heap-oob_10a5a97_1026_42f9d4855547329560f385768de2f3fb.wtv

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit cc5e5548df)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/wtvdec.c

@@ -1033,7 +1033,7 @@ static int read_header(AVFormatContext *s)
                     while (1) {
                         uint64_t frame_nb = avio_rl64(pb);
                         uint64_t position = avio_rl64(pb);
-                        while (frame_nb > e->size && e <= e_end) {
+                        while (e <= e_end && frame_nb > e->size) {
                             e->pos = last_position;
                             e++;
                         }