Fix segfault when X starts

This patch potentially fixes bug https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=884934

System log entries when this bug occurs: kernel: xauth[16729]: segfault at 1 ip 00007f51f517f5a5 sp 00007ffdec846568 error 4 in libc-2.31.so[7f51f5102000+144000] kernel: Code: bc d1 f3 0f 7f 27 f3 0f 7f 6f 10 f3 0f 7f 77 20 f3 0f 7f 7f 30 49 83 c0 0f 49 29 d0 48 8d 7c 17 31 e9 8f 0b 00 00 66 0f ef c0 0f 6f 0e f3 0f 6f 56 10 66 0f 74 c1 66 0f d7 d0 49 83 f8 11 0f

This bug happens when function get_address_info() in gethost.c is called with a display name without forward slash, for example 'myhost.mydomain:0'