Fix an integer underflow in build_read_multi_rsp

When p_buf->len is mtu - 1 and p_cmd->multi_req.variable_len
evaluates to true, integer underflow is triggered
in the following line, resulting OOB access.

```
 len = p_rsp->attr_value.len - (total_len - mtu);
```

Bug: 273874525
Test: manual
Ignore-AOSP-First: security
Tag: #security

Merged-In: Ia60dd829ff9152c083de1f4c1265bb3ad595dcc4
Change-Id: Ia60dd829ff9152c083de1f4c1265bb3ad595dcc4