Detect premature EOF while parsing corrupted key packets.

This helps in the case of an unknown key algorithm with a corrupted
packet which claims a longer packet length. This used to allocate the
announced packet length and then tried to fill it up without detecting
an EOF, thus taking quite some time. IT is easy to fix, thus we do
it. However, there are many other ways to force gpg to use large
amount of resources; thus as before it is strongly suggested that the
sysadm uses ulimit do assign suitable resource limits to the gpg
process. Suggested by Timo Schulz.