In setfval(), ensure that arithmetic never yields a negative zero

result.  Now {echo | awk '{print -1 * 0}'} prints "0" instead of "-0".

Fixes PR 36831.  Discussed in tech-userlevel.

dist/nawk/tran.c

@@ -285,6 +285,7 @@ Awkfloat setfval(Cell *vp, Awkfloat f)	/* set float val of a Cell */
 {
 	int fldno;
 
+	f += 0.0;		/* normalise negative zero to positive zero */
 	if ((vp->tval & (NUM | STR)) == 0) 
 		funnyvar(vp, "assign to");
 	if (isfld(vp)) {