The gcrypt definition of `bn_new` used to use the return statement
on errors, with an AVERROR return value, regardless of the signature
of the function where the macro is used - it is called in
`dh_generate_key` and `ff_dh_init` which return pointers. As a result,
compiling with gcrypt and the ffrtmpcrypt protocol resulted in an
int-conversion warning. GCC 14 may upgrade these to errors [1].

This patch fixes the problem by changing the macro to remove `AVERROR`
and instead set `bn` to null if the allocation fails. This is the
behaviour of all the other `bn_new` implementations and so the result is
already checked at all the callsites. AFAICT, this should be the only
change needed to get ffmpeg off Fedora's naughty list of projects with
warnings which may be upgraded to errors in GCC 14 [2].

[1]: https://gcc.gnu.org/pipermail/gcc/2023-May/241264.html
[2]: https://www.mail-archive.com/devel@lists.fedoraproject.org/msg196024.htmlSigned-off-by: Frank Plowman <post@frankplowman.com>
Signed-off-by: Martin Storsjö <martin@martin.st>