lib: talloc: Make it clear that talloc_get_size(NULL) returns 0.

lib: talloc: Make it clear that talloc_get_size(NULL) returns 0.

This *isn't* a behavior change, as the previous code could potentially
return the size of null_context, which (currently) is defined as
a named talloc region of ZERO size, but this makes it very clear
what the ABI behavior should be.

Signed-off-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>
Autobuild-User(master): Stefan Metzmacher <metze@samba.org>
Autobuild-Date(master): Thu Jan 12 19:23:25 CET 2017 on sn-devel-144