util/error: Fix use-after-free errors reported by Coverity

Fix use-after-free errors in the code path that called error_handle(). A
call to error_handle() will now either free the passed Error 'err' or
assign it to '*errp' if '*errp' is currently NULL. This ensures that 'err'
either has been freed or is assigned to '*errp' if this function returns.
Adjust the two callers of this function to not assign the 'err' to '*errp'
themselves, since this is now handled by error_handle().

Fixes: commit 3ffef1a55ca3 ("error: add global &error_warn destination")
Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20230406154347.4100700-1-stefanb@linux.ibm.com

util/error.c

@@ -46,6 +46,10 @@ static void error_handle(Error **errp, Error *err)
     }
     if (errp == &error_warn) {
         warn_report_err(err);
+    } else if (errp && !*errp) {
+        *errp = err;
+    } else {
+        error_free(err);
     }
 }
 
@@ -76,7 +80,6 @@ static void error_setv(Error **errp,
     err->func = func;
 
     error_handle(errp, err);
-    *errp = err;
 
     errno = saved_errno;
 }
@@ -289,11 +292,6 @@ void error_propagate(Error **dst_errp, Error *local_err)
         return;
     }
     error_handle(dst_errp, local_err);
-    if (dst_errp && !*dst_errp) {
-        *dst_errp = local_err;
-    } else {
-        error_free(local_err);
-    }
 }
 
 void error_propagate_prepend(Error **dst_errp, Error *err,