channel: make variables more local
channel: make variables more local

By making 'data' and 'data_len' more local in several places in
this file it will be easier to spot how they are used and we'll
get less risks to accidentally do bad things with them.

libssh2/libssh2,libssh2/libssh2,libssh2/libssh2,libssh2/libssh2