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mpdiv: fix dividing 0 by a small power of two

--- a/sys/src/libmp/port/mpdiv.c

+++ b/sys/src/libmp/port/mpdiv.c

@@ -22,7 +22,9 @@

 	// division by one or small powers of two

 	if(divisor->top == 1 && (divisor->p[0] & divisor->p[0]-1) == 0){

-		vlong r = (vlong)dividend->sign * (dividend->p[0] & divisor->p[0]-1);

+		vlong r = 0;

+		if(dividend->top > 0)

+			r = (vlong)dividend->sign * (dividend->p[0] & divisor->p[0]-1);

 		if(quotient != nil){

 			sign = divisor->sign;

 			for(s = 0; ((divisor->p[0] >> s) & 1) == 0; s++)