ast-build: make sure InterpDef is freeable

With the following two rules:

    InterpretDecl   :       INTERPRET InterpretMatch OBRACE
                                VarDeclList
                            CBRACE SEMI
                            { $2->def = $4; $$ = $2; }
                    ;

    InterpretMatch  :       KeySym PLUS Expr
                            { $$ = InterpCreate($1, $3); }
                    |       KeySym
                            { $$ = InterpCreate($1, NULL); }
                    ;

And the fact that InterpCreate doesn't initialize ->def, if the
VarDeclList fails, the %destructor tries to recursively free the
uninitialized ->def VarDef. So always initialize it.

That was the only problematic code in the parser for %destructor (I'm
pretty sure).

Signed-off-by: Ran Benita <ran234@gmail.com>