talloc: fix compiler warning

This avoids the following warning when using:

CFLAGS="-O3 -g -fstrict-overflow -Wstrict-overflow=5"

../talloc.c: In Funktion »talloc_is_parent«:
../talloc.c:2658:21: Warnung: assuming signed overflow does not occur when
changing X +- C1 cmp C2 to X cmp C1 +- C2 [-Wstrict-overflow]

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>