wireshark: Don't leak column strings

wireshark: Don't leak column strings

One of the problems of using val_to_str() is that it may return a
const string from given table ('vs'), OR return an allocated one.
Since the caller has no idea which case it is, it resides to safe
option and don't free returned string. But that might lead to a
memleak. This behaviour is fixed with wireshark-4.6.0 and support
for it will be introduced soon. But first, make vir_val_to_str()
behave like fixed val_to_str() from newer wireshark: just always
allocate the string.

Now, if val_to_str() needs to allocate new memory it obtains
allocator by calling wmem_packet_scope() which is what we may do
too.

Hand in hand with that, we need to free the memory using the
correct allocator, hence wmem_free(). But let's put it into a
wrapper vir_wmem_free() because just like val_to_str(), it'll
need additional argument when adapting to new wireshark.

Oh, and freeing the memory right after col_add_fstr() is safe as
it uses vsnprintf() under the hood to format passed args.

One last thing, the wmem.h file used to live under epan/wmem/ but
then in v3.5.0~240 [1] was moved to wsutil/wmem/.

1: https://gitlab.com/wireshark/wireshark/-/commit/7f9c1f5f92c131354fc8b2b88d473706786064c0
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>