Added test for NULL in string.format("%p")

ISO C states that standard library functions should not be called
with NULL arguments, unless stated otherwise. 'sprintf' does not
state otherwise, and it doesn't hurt to be on the safe side.

lstrlib.c

@@ -1271,6 +1271,8 @@ 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' */
           nb = l_sprintf(buff, maxitem, form, p);
           break;
         }