Genrate warning when NTA not subclass of ASTNode

[Alfred Åkesson]

Example following code will generate NullPointerException.

Ast:

A;

Jrag:

aspect Test {
    syn nta interfaceB A.getA();
    interface interfaceB {}
}

Generate a error insted.

[Jesper Öqvist]

An error message for this error is already implemented in JastAdd, however it is never shown because JastAdd crashes while building the parent map:

java.lang.NullPointerException
    at org.jastadd.ast.AST.Grammar.parentMap_compute(Grammar.java:2171)
    at org.jastadd.ast.AST.Grammar.parentMap(Grammar.java:2139)
    at org.jastadd.ast.AST.ASTDecl.parents_compute(ASTDecl.java:4574)
    at org.jastadd.ast.AST.ASTDecl.parents(ASTDecl.java:4562)
    at org.jastadd.ast.AST.ASTDecl.isRootNode(ASTDecl.java:4964)
    at org.jastadd.ast.AST.Grammar.roots_compute(Grammar.java:1986)
    at org.jastadd.ast.AST.Grammar.roots(Grammar.java:1972)
    at org.jastadd.ast.AST.Grammar.collect_contributors_Grammar_problems(Grammar.java:2479)
    at org.jastadd.ast.AST.Grammar.survey_Grammar_problems(Grammar.java:1575)
    at org.jastadd.ast.AST.Grammar.problems_compute(Grammar.java:2462)
    at org.jastadd.ast.AST.Grammar.problems(Grammar.java:2445)
    at org.jastadd.JastAdd.compile(JastAdd.java:210)
    at org.jastadd.JastAdd.compile(JastAdd.java:129)
    at org.jastadd.JastAdd.main(JastAdd.java:114)

[Jesper Öqvist]

Fix crash in parentMap computation

The parentMap attribute would crash if the return type of a synthesized NTA declared in the abstract grammar was not an AST class.

fixes #280 (bitbucket)

→ <<cset 7581c82f11a7>>