Fix bug causing crash on Windows

A reference was being kept to a field of an element of a vector that had been swapped with another vector. Whether the original element still exists at the original memory location, preserving references, is implementation dependant and should not be relied upon.

Author: NathanJPhillips

src/ansi-c/c_typecheck_expr.cpp

@@ -1244,7 +1244,10 @@ void c_typecheck_baset::typecheck_expr_index(exprt &expr)
 
   make_index_type(index_expr);
 
-  const typet &final_array_type=follow(array_expr.type());
+  // array_expr is a reference to one of expr.operands(), when that vector is
+  // swapped below the reference is no longer valid. final_array_type exists
+  // beyond that point so can't be a reference
+  const typet final_array_type = follow(array_expr.type());
 
   if(final_array_type.id()==ID_array ||
      final_array_type.id()==ID_vector)