Why we don't panic in PageGetExactFreeSpace

The PageGetExactFreeSpace function contains following code:

00486     space = (int) ((PageHeader) page)->pd_upper -
00487         (int) ((PageHeader) page)->pd_lower;
00488
00489     if (space < 0)
00490         return 0;


It seems to me that we should panic that data are overlaped instead of return 
zero. See PageHeaderIsValid for reference.
Any comments?
    Zdenek

-- 
Zdenek Kotala              Sun Microsystems
Prague, Czech Republic     http://sun.com/postgresql