conncache: silence `-Wnull-dereference` on gcc 14 RISC-V 64

conncache: silence `-Wnull-dereference` on gcc 14 RISC-V 64

A false positive that appeared after a recent patch for no reason.

Seen in curl-for-win unity native Linux builds on debian:testing and
debian:trixie with gcc 14.3.0 and 14.2.0 respectively:
```
-- curl version=[8.17.1-DEV]
-- The C compiler identification is GNU 14.2.0
-- Cross-compiling: Linux/x86_64 -> Linux/riscv64
[...]
lib/conncache.c: In function 'Curl_cpool_conn_now_idle':
lib/conncache.c:539:11: error: null pointer dereference [-Werror=null-dereference]
  539 |   if(!data->multi->maxconnects) {
      |       ~~~~^~~~~~~
```
Ref: https://github.com/curl/curl-for-win/actions/runs/19111497271/job/54609512969#step:3:5788

```
-- The C compiler identification is GNU 14.3.0
```
Ref: https://github.com/curl/curl-for-win/actions/runs/19111497271/job/54609512899#step:3:5801

Patch confirmed silencing:
https://github.com/curl/curl-for-win/actions/runs/19112580362/job/54613288202

Follow-up to fbc4d59151dc4a56052f3a92da3682dc97b32148 #19271

Closes #19378