(for 4.9.3) CVE-2018-14466/Rx: fix an over-read bug

In rx_cache_insert() and rx_cache_find() properly read the serviceId
field of the rx_header structure as a 16-bit integer. When those
functions tried to read 32 bits the extra 16 bits could be outside of
the bounds checked in rx_print() for the rx_header structure, as
serviceId is the last field in that structure.

This fixes a buffer over-read discovered by Bhargava Shastry,
SecT/TU Berlin.

Add a test using the capture file supplied by the reporter(s).