I was looking at a performance regression with some threading changes 
I'm working on and spotted this trivial cleanup.

in bitmap_find_bit:

  /* `element' is the nearest to the one we want.  If it's not the one we
     want, the one we want doesn't exist.  */
  head->current = element;
  head->indx = element->indx;
  if (element != 0 && element->indx != indx)
    element = 0;

ELEMENT will always be non-NULL at the conditional as it was 
dereferenced in the prior statement.  And if we look up further (not 
shown here), we can deduce that ELEMENT will always be non-NULL at the 
dereference point as well.

Things have been like this since the introduction of bitmap.c in 1997.

VRP will catch this, but its kind of silly to not clean this nit up at 
the source level.

Bootstrapped and regression tested on x86_64 linux.

OK for the trunk?