Index expression should used signed_size_type rather than size_type

I based my code in the previous commit on this code, but then found it
caused a type error. This leads me to believe that while this current
code path may not be called in normal usage, it is in fact a latent bug.
I believe this change is correct.

Author: jezhiggins

src/analyses/variable-sensitivity/write_stack.cpp

@@ -39,10 +39,10 @@ write_stackt::write_stackt(
   if(expr.type().id() == ID_array)
   {
     // We are assigning an array to a pointer, which is equivalent to assigning
-    // the first element of that arary
+    // the first element of that array
     // &(expr)[0]
     construct_stack_to_pointer(
-      address_of_exprt(index_exprt(expr, from_integer(0, size_type()))),
+      address_of_exprt(index_exprt(expr, from_integer(0, signed_size_type()))),
       environment,
       ns);
   }