tcpdump: plug a memory leak.

tcpdump: plug a memory leak.

If the -V flag is used, and not all files in the -V file have the same
link-layer type, when the filter is recompiled for a new link-layer
type, the old filter program is leaked.  Free the old filter before
compiling the new filter.

(cherry picked from commit 9904bbbd9ed020931d55ce6a4aed8a1dfc3d3841)