Stdio.Buffer: Handle UNDEFINED nbytes in input_from().

The nbytes argument may be UNDEFINED (typically when the once argument
has been specified). Handle it as if nbytes hasn't been specified.

Thanks to Stephen R. van den Berg <srb@cuci.nl> for the report.

Fixes some of [LysLysKOM 20998397]/[Pike mailinglist 14064].
Stdio.Buffer: Fix NOP call.