main: guard against rnd(0) in rndmapname

starting ac_client from the wrong working directory, where
packages/maps/official/*.cgz globs to an empty list, we
trigger a SIGFPE because of rnd(0):

(gdb) r
Program received signal SIGFPE, Arithmetic exception.
0x000000000044d37d in rndmapname () at main.cpp:897
897	    char *map = newstring(maps[rnd(maps.length())]);
(gdb) bt