Fix: fs.open callback did not fire

Problem: Omitting the mode parameter causes the provided callback
parameter to never fire. This was originally fixed in 6078c37b and
later broken in 5f2e9093.

Solution: Overwriting the value of a parameter also overwrites the
reference in the arguments object. This patch works arround this
fact by not touching the mode parameter until a reference to the
callback has been established.