Fix jsonpath .split_part() to honor silent mode

Fix jsonpath .split_part() to honor silent mode

The jsonpath .split_part() method passed its field-position argument
through numeric_int4(), that can fail hard if called directly.

This commit switches the code to use numeric_int4_safe() with an error
context for soft reporting, so as the overflow and zero field-position
cases can be handled in silent mode.

Oversight in bd4f879a9cdd.

Author:  Chao Li <li.evan.chao@gmail.com>
Reviewed-by: Nazir Bilal Yavuz <byavuz81@gmail.com>
Discussion: https://postgr.es/m/FCF996D0-580B-431C-8DE1-A540C58E444C@gmail.com