When a new display filter is to be applied, don't set "cf.dfilter" or

When a new display filter is to be applied, don't set "cf.dfilter" or
"cf.dfcode" if the new filter doesn't compile, because the filter
currently in effect will be the one that was last applied - just free up
the text of the new filter, and whatever memory was allocated for the
new filter code.

This means we allocate a new dfilter when a new filter is to be applied,
rather than recycling stuff from the old filter, as we want the old
filter code to remain around if the new filter doesn't compile.

This means that "cf.dfilter" and "cf.dfcode" will be null if there's no
filter in effect.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@803 f5534014-38df-0310-8fa8-9805f1628bb7