Patch from Gerrit: hostlist___hostrange_string__hostlist_next.diff

_hostrange_string()/hostlist_next(): test for overrun

This lets the two functions _hostrange_string/hostlist_next return NULL in case
there is an overflow condition.

Two possible overflow conditions were analyzed:
 * if len <= MAXHOSTNAMELEN + 15 and the '=' is the case, then len++ means
   that buf[len] = '\0' is the same as buf[MAXHOSTNAMELEN + 16], which would
   cause a segmentation violation - fixed by checking length beforehand;
 * also checks the return value of the snprintfs,
 * and requires 'dims' as minimum suffix length.