h264: disable assert on current_picture_ptr being null.

It is possible in various error pathes as well as gap handling
that this has already been allocated. Its not clear why that
would be a problem with the current code, thus disable the
assert to avoid common assert failure when asserts are enabled.
Signed-off-by: NMichael Niedermayer <michaelni@gmx.at>

libavcodec/h264.c

@@ -2836,7 +2836,7 @@ static int decode_slice_header(H264Context *h, H264Context *h0)
             }
         } else {
             /* Frame or first field in a potentially complementary pair */
-            assert(!s0->current_picture_ptr);
+//             assert(!s0->current_picture_ptr);
             s0->first_field = FIELD_PICTURE;
         }