common: fix ubsan trigger in param test.

```
common/test/run-param.c:381:8: runtime error: applying zero offset to null pointer
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior common/test/run-param.c:381:8
```

Probably because CI now on 24.04, so more recent clang.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

Author: rustyrussell

common/test/run-param.c

@@ -378,7 +378,7 @@ static void bad_programmer(void)
 
 	paramcheck_assert_failed = false;
 	param(cmd, j->buffer, j->toks,
-	      p_req("u64", (param_cbx) NULL, NULL), NULL);
+	      p_req("u64", param_u64, NULL), NULL);
 	assert(paramcheck_assert_failed);
 
 	/* Add required param after optional */