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

1 files changed, 2 insertions, 2 deletions

diff --git a/sys/x86/x86/ucode_subr.c b/sys/x86/x86/ucode_subr.c
index 53d7cfc06769..9536ed7b8f7b 100644
--- a/sys/x86/x86/ucode_subr.c
+++ b/sys/x86/x86/ucode_subr.c
@@ -98,11 +98,11 @@ ucode_amd_find(const char *path, uint32_t signature, uint32_t *revision,
     const uint8_t *fw_data, size_t fw_size, size_t *selected_sizep)
 {
 	const amd_10h_fw_header_t *fw_header;
-	const amd_10h_fw_header_t *selected_fw;
+	const amd_10h_fw_header_t *selected_fw = NULL;
 	const equiv_cpu_entry_t *equiv_cpu_table;
 	const section_header_t *section_header;
 	const container_header_t *container_header;
-	size_t selected_size;
+	size_t selected_size = 0;
 	uint16_t equiv_id;
 	int i;