idmap_autorid: fix status code when trying to load range for an invalid input

idmap_autorid: fix status code when trying to load range for an invalid input

The "sid" input needs to be verified (it can currently be a SID or "ALLOC").
When handing in string that is valid for other kinds of records,
but not for the SID[#IDX]-->RANGE direction of mappings, like for instance
a range number, then we get "NT_STATUS_INTERNAL_DB_CORRUPTION" because
parse records finds the record, but it does not have the expected size...

This patch fixes this problem by pre-validating the input before fetching
the record from the database.

Signed-off-by: Michael Adam <obnox@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>