Improve error checking in get_all()
Improve error checking in get_all()

After getting a too big value via ERANGE, making the call with the
proper length could still fail due to other issues, and this check is
missing. A negative value here would be changed into something weird
in the realloc call (which takes size_t), so the error reporting would
be bogus.

iustin/pyxattr,iustin/pyxattr