If "pcap_compile()" fails to compile an empty filter string (because,

If "pcap_compile()" fails to compile an empty filter string (because,
for example, the libpcap code generator doesn't support the link-layer
type for the capture), "dfilter_compile()" will succeed but return a
null rfcode pointer.

In that case, instead of telling people that it looks like a valid
display filter (which it does, but it also looks like a complete list of
all the Basque words likely to be known by Hammurabi :-)), and then
crashing when we try to "free" that non-existent dfilter code, we just
report it as a "sorry, couldn't compile that capture filter.

svn path=/trunk/; revision=9912