Restore handling of -0 in the C field of lines in line_construct().

Commit a7dc63d904 inadvertedly removed this bit originally introduced
by 43fe90f66a, causing regression test failures on some platforms,
due to producing {1,-1,-0} instead of {1,-1,0}.

src/backend/utils/adt/geo_ops.c

@@ -1024,6 +1024,9 @@ line_construct(LINE *result, Point *pt, float8 m)
 		result->A = m;
 		result->B = -1.0;
 		result->C = pt->y - m * pt->x;
+		/* on some platforms, the preceding expression tends to produce -0 */
+		if (result->C == 0.0)
+			result->C = 0.0;
 	}
 }