minor: update checks (related to checkstyle#3095)

Author: nrmancuso

sevntu-checks/src/main/java/com/github/sevntu/checkstyle/checks/coding/AvoidDefaultSerializableInInnerClassesCheck.java

@@ -80,7 +80,8 @@ public int[] getRequiredTokens() {
 
     @Override
     public void visitToken(DetailAST detailAST) {
-        final boolean topLevelClass = detailAST.getParent() == null;
+        final boolean topLevelClass =
+                detailAST.getParent().getType() == TokenTypes.COMPILATION_UNIT;
         if (!topLevelClass && isSerializable(detailAST)
                 && !isStatic(detailAST)
                 && !hasSerialazableMethods(detailAST)) {

sevntu-checks/src/main/java/com/github/sevntu/checkstyle/checks/design/InnerClassCheck.java

@@ -81,7 +81,7 @@ public void visitToken(DetailAST ast) {
     @Override
     public void leaveToken(DetailAST ast) {
         // Is this a root class
-        if (ast.getParent() == null) {
+        if (ast.getParent().getType() == TokenTypes.COMPILATION_UNIT) {
             rootClass = true;
         }
     }

sevntu-checks/src/main/java/com/github/sevntu/checkstyle/checks/design/PublicReferenceToPrivateTypeCheck.java

@@ -338,7 +338,8 @@ private boolean isExtendsOrImplementsSmth(DetailAST classOrInterfaceDefAst) {
      */
     private static boolean
             isDefinedInTopLevelClass(DetailAST methodOrFieldDefAst) {
-        return methodOrFieldDefAst.getParent().getParent().getParent() == null;
+        return methodOrFieldDefAst.getParent().getParent()
+                .getParent().getType() == TokenTypes.COMPILATION_UNIT;
     }
 
 }

sevntu-checks/src/main/java/com/github/sevntu/checkstyle/checks/sizes/LineLengthExtendedCheck.java

@@ -206,8 +206,7 @@ public int[] getRequiredTokens() {
 
     @Override
     public void visitToken(DetailAST ast) {
-        if (ast.getParent() != null
-                && ast.getParent().getType() == TokenTypes.OBJBLOCK
+        if (ast.getParent().getType() == TokenTypes.OBJBLOCK
                 || ast.getType() == TokenTypes.CLASS_DEF) {
             final int mNumberOfLine = ast.getLineNo();
             final DetailAST endOfIgnoreLine = ast.findFirstToken(TokenTypes.SLIST);