h264: Remove an assert on current_picture_ptr being null

It is possible in various error paths as well as gap handling
that this has already been allocated. It is not clear why that
would be a problem with the current code, thus disable the
assert to avoid a common assert failure when asserts are enabled.

Signed-off-by: Martin Storsjö <martin@martin.st>
(cherry picked from commit 5e997688f8)
Signed-off-by: Diego Biurrun <diego@biurrun.de>

libavcodec/h264.c

@@ -3109,7 +3109,6 @@ static int decode_slice_header(H264Context *h, H264Context *h0){
 
         } else {
             /* Frame or first field in a potentially complementary pair */
-            assert(!s0->current_picture_ptr);
             s0->first_field = FIELD_PICTURE;
         }