Ignore sizes which do not fit into off_t

Ignore sizes which do not fit into off_t

It is possible to handle entries and files with sizes which do not fit
into off_t of the current system (Windows always has 32 bit off_t and
32 bit systems without large file support also have 32 bit off_t).

Set sizes to 0 in such cases. The fstat system call would return -1 and
set errno to EOVERFLOW, but that's not how archive_entry_set_size acts.
It would simply ignore negative values and set the size to 0.

Actual callers of archive_entry_stat from foreign projects seem to not
even check for NULL return values, so let's try to handle such cases as
nice as possible.

Affects mtree's checkfs option as well (Windows only, 32 bit systems
would simply fail in fstat/stat).

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>