ecpg: Fix return code for overflow in numeric conversion
The decimal conversion functions dectoint and dectolong are documented
to return ECPG_INFORMIX_NUM_OVERFLOW in case of overflows, but always
returned -1 on all errors due to incorrectly checking the returnvalue
from the PGTYPES* functions.
Author: Aidar Imamov <a.imamov@postgrespro.ru>
Discussion: https://postgr.es/m/
54d2b53327516d9454daa5fb2f893bdc@postgrespro.ru