res_pjsip_logger.c: correct the return value checks when writing to pcap

res_pjsip_logger.c: correct the return value checks when writing to pcap
files

fwrite() does return the number of elements written and not the
number of bytes. However asterisk is currently comparing the return
value to the size of the written element what means that asterisk logs
five WARNING messages on every packet written to the pcap file.

This patch changes the code to check for the correct value, which will
always be 1.

ASTERISK-28921 #close

Change-Id: I2455032d9cb4c5a500692923f9e2a22e68b08fc2