fix division by zero error

Author: brittnilorton

Contrib/CCCS/AlgebraicLiteracy/Equations_of_Lines/Parallel_Perp_1.pg

@@ -63,7 +63,7 @@ if ( $chooser == 0) {
    $compare = "negative reciprocals";
    $answer = "perpendicular";
 } else {
-   do { $B2 = random(-20,20); } until (($B2 != $B2Para) and ($B2 != $B2Perp));
+   do { $B2 = non_zero_random(-20,20); } until (($B2 != $B2Para) and ($B2 != $B2Perp));
    $popup = PopUp(["?","Parallel", "Perpendicular","Neither"], "Neither");
    $compare = "different but not negative reciprocals";
    $answer = "neither parallel nor perpendicular";