x86/ucode: fix gcc uninitialised warningHEADmain

This fixes the case where selected_size is never set to anything
in the loop.  Whilst here, also set selected_fw to NULL so the case
of "no firmware" correctly sets everything to NULL/0.

```
  --- ucode_subr.o ---
  /workspace/src/sys/x86/x86/ucode_subr.c: In function 'ucode_amd_find':
  /workspace/src/sys/x86/x86/ucode_subr.c:237:25: warning: 'selected_size' may be used uninitialized [-Wmaybe-uninitialized]
    237 |         *selected_sizep = selected_size;
        |         ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~
/  workspace/src/sys/x86/x86/ucode_subr.c:105:16: note: 'selected_size' was declared here
    105 |         size_t selected_size;
        |                ^~~~~~~~~~~~~
```

Reviewed by:	emaste
Differential Revision:	https://reviews.freebsd.org/D55439

0 files changed, 0 insertions, 0 deletions