virBitmapIsBitSet: Allow NULL bitmap

virBitmapIsBitSet: Allow NULL bitmap

The virBitmapIsBitSet API is a permissive one which returns false when
the bit is not set or is out of range. We can do the same if the bitmap
is NULL to aid certain situations when this can happen, but we don't
want to add extra checks.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>