bgpd: Optimize the way parsing communities if no community alias exists

bgpd: Optimize the way parsing communities if no community alias exists

If at least one community alias is configured, then let's do the work,
otherwise we don't need to spend time on splitting stuff and creating
a new string.

This should improve the performance.

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>