expr2bits: bit strings for null pointers must have pointer width

The order of constructor arguments was wrong, resulting in 48 copies
(the ASCII index of '0') instead of bit-width copies, being used.

Author: tautschnig

src/util/simplify_utils.cpp

@@ -201,6 +201,13 @@ optionalt<exprt> bits2expr(
     type.id() == ID_c_bit_field || type.id() == ID_pointer ||
     type.id() == ID_bv || type.id() == ID_c_bool)
   {
+    if(
+      type.id() == ID_pointer && config.ansi_c.NULL_is_zero &&
+      bits.find('1') == std::string::npos)
+    {
+      return null_pointer_exprt(to_pointer_type(type));
+    }
+
     endianness_mapt map(type, little_endian, ns);
 
     std::string tmp = bits;
@@ -410,7 +417,7 @@ expr2bits(const exprt &expr, bool little_endian, const namespacet &ns)
     else if(type.id() == ID_pointer)
     {
       if(value == ID_NULL && config.ansi_c.NULL_is_zero)
-        return std::string('0', to_bitvector_type(type).get_width());
+        return std::string(to_bitvector_type(type).get_width(), '0');
       else
         return {};
     }