Fix: flt.utils.muxer: reset is_ended variable after each iteration

Problem
-------
Whenever an upstream message iterator returns STATUS_END, we remove it
from the array used to order the messages. The variable `is_ended` is
used to record if a upstream message iterator is ended and must be
removed. This variable is only set to false _before_ the loop. If the
first upstream message iterator in the array needs to be removed, all
the following iterators will be mistakenly removed.

Solution
--------
The `is_ended` variable must be reset to false at each iteration so that
only the upstream message iterator that is actually ended is removed from
the array.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>