x86/ucode: fix gcc uninitialised warning
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