Fix use-after-free warning

Fix use-after-free warning

When building, I get the following warning:

```bash
$ ninja
[18/71] Compiling C object lib/libfuse3.so.3.14.1.p/modules_iconv.c.o
../lib/modules/iconv.c: In function ‘iconv_convpath’:
../lib/modules/iconv.c:85:38: warning: pointer ‘newpath’ may be used after ‘realloc’ [-Wuse-after-free]
   85 |                         p = tmp + (p - newpath);
      |                                   ~~~^~~~~~~~~~
../lib/modules/iconv.c:80:31: note: call to ‘realloc’ here
   80 |                         tmp = realloc(newpath, newpathlen + 1);
      |                               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
[71/71] Linking target example/passthrough_hp
```

It's a false positive,  I thinks.  But it's also easy to silence this
warning with a small refactor.