Attempt to resolve crashiness by clearing pendingInvalidations more aggr...

...essively.

I already discussed this a little with Niko earlier today (see [log](http://logbot.glob.com.au/?c=mozilla%23pjs#c15513)).  I mostly want either Shu or Niko sanity-check my reasoning that this is a conservative change.

(My working hypothesis is that relying on parallel() to reset the state of pendingInvalidations each time through the loop is a bad idea, as Niko alluded to in the irc log.)

The JS_ASSERT is just a side-note; if someone objects I will remove it.  (It was leftover from my attempt to check that a supposed rep-invariant was established, and then I did binary search to find out where the rep invariant was being broken.)