CtExecutableReferenceImpl may throw a NullPointerException in getDeclaration

[msteinbeck]

I found that CtExecutableReferenceImpl#getDeclaration may throw a NullPointerException in noclasspath mode. Unfortunately, I can not provide a short example but it definitely occurs. The following snippet shows it's implementation:

@Override
@SuppressWarnings("unchecked")
public CtExecutable<T> getDeclaration() {
    return getCtExecutable(getDeclaringType().getDeclaration());
}

As we can see, getDeclaration calls getDeclaringType() which, according to it's JavaDoc, may return null. By calling getDeclaringType().getDeclaration() a NullPointerException is throws if getDeclaringType() is null.

[msteinbeck]

An example is given in PR.