Make libssh2_packet_read() use a static buffer to avoid unnecessary
alloc and free calls.

Since the cipher and mac layers aren't runtime extensible (yet)
and we know neither of these works in sizes greater than 32,
we can safely set aside a 64 byte block of data on the stack
for crypting and hashing.

This will make a big difference during quasi-non-blocking reads
where libssh2_packet_read() is polled repeatedly.
2 files changed