pjsip_distributor.c: Use correct rdata info access method.

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: Iafe8b01242b7deb0ebfdc36685e21374a43936d2