common-lib: Update error check for new string conversion wrapper

common-lib: Update error check for new string conversion wrapper

The new string conversion wrappers detect and flag errors
which occured during the string to integer conversion.
Those modifications required an update of the callees
error checks.

Signed-off-by: Swen Schillig <swen@linux.ibm.com>
Reviewed-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Christof Schmitt <cs@samba.org>