libm: remainder: make sure x is zero

Make sure the entirety of x is zero before flipping the sign bit.
Otherwise the sign would be wrong for small values of x when x is
negative and |n*y| > |x|

Reported by:	alfredo
PR:		251091
Reviewed by:	kargl
MFC after:	3 days
Differential Revision: https://reviews.freebsd.org/D53023

lib/msun/src/e_remainder.c

@@ -64,8 +64,8 @@ remainder(double x, double p)
 		if(x>=p_half) x -= p;
 	    }
 	}
-	GET_HIGH_WORD(hx,x);
+	EXTRACT_WORDS(hx, lx, x);
-	if ((hx&0x7fffffff)==0) hx = 0;
+	if (((hx&0x7fffffff)|lx) == 0) hx = 0;
 	SET_HIGH_WORD(x,hx^sx);
 	return x;
 }