fix for readall_s at EOF

This fails on gentoo as seen here: https://github.com/perl6/nqp/issues/133
Now it does not try to read from a file when there are no bytes left.
[test] add test for GH #981, readall at eof

fsils in master, fixed in smoke-me/readall_gentoo_gh981
Merge branch 'smoke-me/readall_gentoo_gh981'

Smoked okay on linux, darwin, win32, cygwin.