When we open a new .while loop, let's not attempt to close out

When we open a new .while loop, let's not attempt to close out
another enclosing .while loop at the same time.
Instead, postpone the closing until the next iteration of ROFF_RERUN.

This prevents one-line constructions like ".while 0 .while 0 something"
and ".while rx .while rx .rr x" (which admittedly aren't particularly
useful) from dying of abort(3), which was a bug tb@ found with afl(1).