Fix DEREF_AFTER_NULL in mm_answer_keyallowed()

[maks-mishin]

After having been compared to a NULL value at monitor.c:1206, pointer 'key' is dereferenced in function auth2_record_key.

Found by RASU JSC.

[djmdjm]

key=NULL is impossible here as sshkey_froms() won't return a NULL key. I'll look at removing the unnecessary key==NULL checks instead