Fix segfault when incrementing whole vector values.

Also removed RefExpr::Eval(Val*) method since it was never called
(Clang emitted warning about this hiding overloaded virtual function
UnaryExpr::Eval(Frame*)) and doesn't appear to be necessary even if it
was called to avoid the default vector handling of UnaryExpr::Eval
(as the comment suggests as the intention).

src/Expr.h

@@ -608,10 +608,6 @@ public:
 	void Assign(Frame* f, Val* v, Opcode op = OP_ASSIGN);
 	Expr* MakeLvalue();
 
-	// Only overridden to avoid special vector handling which doesn't apply
-	// for this class.
-	Val* Eval(Val* v) const;
-
 protected:
 	friend class Expr;
 	RefExpr()	{ }