The expression

   sp - 6 < sp

where sp is a u16 is undefined in C since 'sp - 6' is promoted to int,
and signed overflow is undefined in C.  gcc 4.2 actually warns about it.
Replace with a simpler test.
Signed-off-by: NEric Sesterhenn <snakebyte@gmx.de>
Signed-off-by: NAvi Kivity <avi@qumranet.com>