sched.c: Make not return a sched id of 0.

According to the API doxygen a sched ID of 0 is valid.  Unfortunately, 0
was never returned historically and several users incorrectly coded usage
of the returned sched ID assuming that 0 was invalid.

ASTERISK-25476

Change-Id: Ib19c7ebb44ec9fd393ef6646dea806d4f34e3a20