Limit allocation in the BER decoder to 16 MiB.

* src/ber-decoder.c (MAX_IMAGE_LENGTH): New.
(decoder_next): Limit allcoation to MAX_IMAGE_LENGTH.
(_ksba_ber_decoder_dump, _ksba_ber_decoder_decode): Ditto.

We allocate the image used to allocate BER encoded data from the
provided length in the object. However, this length may be given
arbitrary and we would thus try to allocate huge amounts of
memory (and zero them out since commit 2a9fc56) unless the user has
set an appropriate ulimit. This is not desirable and thus we better
bail out early if a strange (ie. very large object is seen).

That whole table driven parser is a mess.

• Reported-by: Pascal Cuoq <cuoq 'at' trust-in-soft com>
• Signed-off-by: Werner Koch <wk@gnupg.org>