boot: check that ret_version is valid in tpm_log_tagged_event

In a project I'm working on I recently observed a boot failure
with the most recent version of systemd. It seems it is triggered
by bb19b61049 which fixed a bug that now leads to the function
being excuted differently. The code is missing a check if
`*ret_version` is actually valid in the `ret_measured = true`
case.

src/boot/measure.c

@@ -312,7 +312,8 @@ EFI_STATUS tpm_log_tagged_event(
         if (err != EFI_SUCCESS)
                 return err;
 
-        *ret_measured = true;
+        if (ret_measured)
+                *ret_measured = true;
         return EFI_SUCCESS;
 }