[PATCH] i386: make pci_map_single/pci_map_sg warn for zero length.

As suggested by Linus.

This catches driver bugs that could cause corruption on IOMMU architectures.

Also I converted the BUGs to out_of_line_bug()s to save a bit
of text space.

Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>