RAR5 reader: Removed a memory leak in process_head_file

RAR5 reader: Removed a memory leak in process_head_file

The process_head_file function was using memset() to clear the
archive_entry structure. The problem was that this structure could
contain pointers to allocated blocks of memory, and removing those
pointers with memset() resulted in a memory leak.

Switching it in favor of archive_entry_clear() effectively clears the
structure, but also releases any allocated memory blocks. This removes
the memory leak.

The commit also changes the way a temporary archive_entry instance is
being created when skipping a base block after block merge; instead of
directly creating a new instance on the stack, a constructor function
archive_entry_new() is used to ensure the new archive_entry instance is
not in an inconsistent state. This is needed because the fix described
in the first half of this commit message depends on the archive_entry
instance being in a consistent state due to the call of the
archive_entry_clear() function.