FIX EOF detection in PT_IO-based to_xfer_partial implementation.

At least on OpenBSD PT_IO/PIOD_READ_AUXV can return sucessfully without
transferring any bytes.  Arguably a kernel bug, but interpreting this as EOF
seems sensible.

gdb/ChangeLog:

        * inf-ptrace.c (inf_ptrace_xfer_partial): Return TARGET_XFER_EOF
        if a PT_IO ptrace request returns sucessfully but indicates that 0
        bytes were transferred.