break out dfcode from the capture file structure and declare it locally where it...

break out dfcode from the capture file structure and declare it locally where it is needed.
allocate and release the dfcode program as needed instead of having it hang around in the capture file structure.

this will ensure that dfcode will not have longer than se scope lifetime in case we need that property of it later

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@20251 f5534014-38df-0310-8fa8-9805f1628bb7