Fix 2nd compilation in Textmode IDE

This simple program will fail to compile in Textmode IDE on 2nd attempt of compilation.

{$h+}

var t1, t2 : string;

begin

writeln(t1+t2);

end.

Reason is that on 2nd attempt some classes point to memory released after 1st compilation. Fix: simply nullify those dangling classes pointers.

Note. Modifies compiler sources.