[BUG] nat_port_select() returned value not safe

### Description / Steps to reproduce the issue

Returned Value:
* External port number on success; 0 on failure
In some cases，when the return value is 0, it may indicate success.


ipv4_nat_outbound_icmp() use icmp->id as port, but some careless applications may send ICMP request packets with icmp->id == 0, in this case, nat_port_select() will return a port number of 0, this 0 is a selected result, not a failure to find a port.


The return value of 0 is interpreted as an error here, preventing subsequent processes from proceeding.

### On which OS does this issue occur?

[OS: Other]

### What is the version of your OS?

.

### NuttX Version

all

### Issue Architecture

[Arch: arm]

### Issue Area

[Area: Networking]

### Host information

_No response_

### Verification

- [x] I have verified before submitting the report.