pjsip_distributor.c: Use correct rdata info access method (Part 2).

pjsip_distributor.c: Use correct rdata info access method (Part 2).

The pjproject doxygen for rdata->msg_info.info says to call
pjsip_rx_data_get_info() instead of accessing the struct member directly.
You need to call the function mostly because the function will generate
the struct member value if it is not already setup.

Change-Id: I4d519385a577f3e9d9193a88125e493cf17fa799