Patch from Gerrit: 06_hostlist__host_prefix_end.diff

host_prefix_end(): save variable, factor out common expressions

This saves one variable and factors out the condition common
to the if/else branches.06_hostlist__host_prefix_end.diff

 * return type of strlen is unsigned (size_t), hence strlen(hostname) >= 0;
 * both while-loops require idx >= 0, hence -1 is returned
   if strlen(hostname) == 0.