scan-build: Be consistent with a needless check in circuitmux.c

In circuitmux_detach_all_circuits, we check whether an HT iterator
gives us NULL.  That should be impossible for an HT iterator.  But
our checking it has confused scan-build (justly) into thinking that
our later use of HT_NEXT_RMV might not be kosher.  I'm taking the
coward's route here and strengthening the check.  Bugfix on
fd31dd44. (Not a real bug though)