sh: Make sure indexes are positive

The indexes are signed, make sure they are not negative
when we read array elements.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>

arch/sh/boards/mach-r2d/irq.c

@@ -116,7 +116,7 @@ static unsigned char irl2irq[R2D_NR_IRL];
 
 int rts7751r2d_irq_demux(int irq)
 {
-	if (irq >= R2D_NR_IRL || !irl2irq[irq])
+	if (irq >= R2D_NR_IRL || irq < 0 || !irl2irq[irq])
 		return irq;
 
 	return irl2irq[irq];