publickey_packet_receive: avoid junk in returned pointers

clang-analyzer found this risk it would return a non-initialized pointer
in a success case
channel_receive_window_adjust: store windows size always

Avoid it sometimes returning without storing it, leaving calling
functions with unknown content!

Detected by clang-analyzer
userauth_hostbased_fromfile: zero assign to avoid uninitialized use

Detected by clang-analyze