Fixed bug in 'string.format("%p")'

The string "(null)" used for non-collectable values must be printed as a
string, not as a pointer. (Bug introduced in commit e0cbaa50fa).

lstrlib.c

@@ -1271,8 +1271,10 @@ static int str_format (lua_State *L) {
         }
         case 'p': {
           const void *p = lua_topointer(L, arg);
-          if (p == NULL)
-            p = "(null)";  /* NULL not a valid parameter in ISO C 'printf' */
+          if (p == NULL) {  /* avoid calling 'printf' with argument NULL */
+            p = "(null)";  /* result */
+            form[strlen(form) - 1] = 's';  /* format it as a string */
+          }
           nb = l_sprintf(buff, maxitem, form, p);
           break;
         }