crypt.c: Plug memory leak in get_public_key().

crypt.c: Plug memory leak in get_public_key().

If server.users refers to an existing file which is not a ssh public
key, we leak 4 bytes of memory:

	==27302== 4 bytes in 1 blocks are definitely lost in loss record 1 of 8
	==27302==    at 0x402C201: malloc (vg_replace_malloc.c:299)
	==27302==    by 0x8052FF3: para_malloc (string.c:63)
	==27302==    by 0x8066532: get_public_key (crypt.c:151)
	==27302==    by 0x80569D1: user_list_init (user_list.c:90)
	==27302==    by 0x804D74D: parse_config_or_die (server.c:279)
	==27302==    by 0x804C719: server_init (server.c:554)
	==27302==    by 0x804C719: main (server.c:655)

Furtunately, this issue is trivial to fix.