lavc: Move the libtwolame encoder registration to the list for external libraries

This makes sure the default behaviour of using the internal encoder
stays the same regardless if libtwolame is enabled or not (as for
any external library).

This fixes fate-lavf-mpg if libav is built with libtwolame enabled.

CC: libav-stable@libav.org
Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>
xwma: Do not leak on failure path

CC: libav-stable@libav.org
Bug-Id: CID 1087092
lavfi: clean memory on error in ADD_FORMAT()

CC: libav-stable@libav.org
Bug-Id: CID 1250334
libtwolame: prevent a NULL pointer dereference

CC: libav-stable@libav.org
Bug-Id: CID 1250330 / CID 1250335
dvdsubdec: Do not leak on failure path

CC: libav-stable@libav.org
Bug-Id: CID 1198262
vp9: Use the correct upper bound for seg_id

And use a macro to make apparent why the value.

Bug-Id: CID 1108595
qdm2: avoid integer overflow

CC: libav-stable@libav.org
Bug-Id: CID 700555