when the output isn't RETVAL, update the SV instead of replacing it

when the output isn't RETVAL, update the SV instead of replacing it

Previously OUTPUT argument parameters would replace ST(n) instead of
updating it, this meant that the caller's supplied value would not be
updated.

This change means that OUTPUT T_BOOL arguments called RETVAL won't be
handled correctly, but since the OUTPUT didn't work previously for
*any* case, this is a net improvement.