hw/uefi: fix parse_hexstr

Make sure we actually have two input characters available before going
to parse two hex digits.  Fixes one byte buffer overflow of the output
buffer in case the input string has an odd number of characters.

Fixes: CVE-2026-48915
Fixes: 12058948ab ("hw/uefi: add var-service-json.c + qapi for NV vars.")
Reported-by: Feifan Qian <bea1e@proton.me>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-ID: <20260526135948.599148-1-kraxel@redhat.com>

hw/uefi/var-service-json.c

@@ -98,7 +98,7 @@ static void parse_hexstr(void *dest, char *src, int len)
     uint8_t *data = dest;
     size_t i;
 
-    for (i = 0; i < len; i += 2) {
+    for (i = 0; i + 1 < len; i += 2) {
         *(data++) =
             parse_hexchar(src[i]) << 4 |
             parse_hexchar(src[i + 1]);