Ignore ustar size when pax size is present (#2405)

Ignore ustar size when pax size is present (#2405)

When the pax `size` field is present, we should ignore the size value in
the ustar header. In particular, this fixes reading pax archives created
by GNU tar with entries larger than 8GB.

Note: This doesn't impact reading pax archives created by libarchive
because libarchive uses tar extensions to store an accurate large size
field in the ustar header. GNU tar instead strictly follows ustar in
this case, which prevents it from storing accurate sizes in the ustar
header.

Resolves #2404