Enable reading NULL_is_zero from symbol table

This was left commented out ever since introducing it in 5952f6a,
which is reasonable for it required bumping the goto-program version. We
have incremented the version since 2014, and thus it is safe to enable
this now, as also confirmed by the regression test.

Author: tautschnig

regression/goto-instrument/show-symbol-table-json/test.desc

@@ -4,6 +4,7 @@ test.c
 "symbolTable": \{
 "main": \{
 "main::1::x": \{
+"__CPROVER_architecture_NULL_is_zero": \{
 ^EXIT=0$
 ^SIGNAL=0$
 --

src/util/config.cpp

@@ -1259,8 +1259,7 @@ void configt::set_from_symbol_table(
   else
     ansi_c.os=ansi_ct::string_to_os(id2string(string_from_ns(ns, "os")));
 
-  // NULL_is_zero=from_ns("NULL_is_zero");
-  ansi_c.NULL_is_zero=true;
+  ansi_c.NULL_is_zero = unsigned_from_ns(ns, "NULL_is_zero") != 0;
 
   // mode, preprocessor (and all preprocessor command line options),
   // lib, string_abstraction not stored in namespace