fastgetattr & isNull

The fastgetattr() attempts to make provision for the case where isnull
is a NULL pointer, but it doesn't seem to work.  I tried it and got:

relcache.c:494: error: invalid use of void expression
relcache.c:494: error: invalid use of void expression
relcache.c:494: warning: left-hand operand of comma expression has no effect
relcache.c:494: warning: left-hand operand of comma expression has no effect

Changing the fourth argument from NULL to &isnull made the problem go away.

I think we should either fix this so it actually works (if that's even
possible), or rip out the code that tries to cope with it.  That
probably wouldn't produce any measurable speedup, but at least it
might save someone else some head-scratching the next time they're
trying to learn this code.

...Robert