make literal equals methods null-safe

keyext.rusty/src/main/java/org/key_project/rusty/ast/expr/BooleanLiteralExpression.java

@@ -35,7 +35,7 @@ public String toString() {
 
     @Override
     public boolean equals(Object obj) {
-        if (obj.getClass() != this.getClass()) {
+        if (obj == null || obj.getClass() != this.getClass()) {
             return false;
         }
         return value == (((BooleanLiteralExpression) obj).value);

keyext.rusty/src/main/java/org/key_project/rusty/ast/expr/IntegerLiteralExpression.java

@@ -77,7 +77,7 @@ public String toString() {
 
     @Override
     public boolean equals(Object obj) {
-        if (obj.getClass() != this.getClass()) {
+        if (obj == null || obj.getClass() != this.getClass()) {
             return false;
         }
         return value.equals(((IntegerLiteralExpression) obj).value)