kex.c: make sure mlist is not set to NULL

... if the currently unsupported LANG methods are called.
Reported by Coverity CID 89834.
pem.c: make sure there's a trailing zero and b64data is not NULL

... if there is no base64 data between PEM header and footer.
Reported by Coverity CID 89823.
pem.c: fix mixed line-endings introduced with 8670f5da24
session.c: check return value of session_nonblock in debug mode

Reported by Coverity CID 89805.
session.c: check return value of session_nonblock during startup

Reported by Coverity CID 89803.
channel.c: remove logically dead code, host cannot be NULL here

... host cannot be NULL in line 525, because it is always
valid (e.g. at least set to "0.0.0.0") after lines 430 and 431.

Reported by Coverity CID 89807.
userauth.c: make sure that sp_len is positive and avoid overflows

... if the pointer subtraction of sp1 - pubkey - 1 resulted in a
negative or larger value than pubkey_len, memchr would fail.

Reported by Coverity CID 89846.