gpg: Fix DoS while parsing mangled secret key packets.

* g10/parse-packet.c (parse_key): Check PKTLEN before calling mpi_read
et al.

Due to the missing length checks PKTLEN may turn negative. Because
PKTLEN is an unsigned int the malloc in read_rest would try to malloc
a too large number and terminate the process with "error reading rest
of packet: Cannot allocate memory".

• Reported-by: Hanno Böck.
• Signed-off-by: Werner Koch <wk@gnupg.org>