agent: Fix error code check from npth_mutex_init.

* agent/call-pinentry.c (initialize_module_call_pinentry): It's an
error when npth_mutex_init returns non-zero.

Cherry-pick from master commit of:
adce73b86fd49d5bbb8884231a26cc7533d400e2

Actually, initialize_module_call_pinentry is only called once from
main. So, this bug had no harm and having the static variable
INITIALIZED is not needed.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>