[PATCH-for-5.1?] qapi/error: Check format string argument in error_propagate_prepend()

[Philippe Mathieu-Daudé]

error_propagate_prepend() "behaves like error_prepend()", and
error_prepend() uses "formatting @fmt, ... like printf()".
error_prepend() checks its format string argument, but
error_propagate_prepend() does not. Fix that.

This would have catched the invalid format introduced in commit
b98e8d1230f:

    CC      hw/sd/milkymist-memcard.o
  hw/sd/milkymist-memcard.c: In function ‘milkymist_memcard_realize’:
  hw/sd/milkymist-memcard.c:284:70: error: format ‘%s’ expects a matching ‘char 
*’ argument [-Werror=format=]
    284 |         error_propagate_prepend(errp, err, "failed to init SD card: 
%s");
        |                                                                     ~^
        |                                                                      |
        |                                                                      
char *

Fixes: 4b5766488f ("Fix use of error_prepend() with &error_fatal, &error_abort")
Inspired-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
---
 include/qapi/error.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/include/qapi/error.h b/include/qapi/error.h
index 7932594dce..eeeef1a34d 100644
--- a/include/qapi/error.h
+++ b/include/qapi/error.h
@@ -381,6 +381,7 @@ void error_propagate(Error **dst_errp, Error *local_err);
  *     error_propagate(dst_errp, local_err);
  * Please use ERRP_GUARD() and error_prepend() instead when possible.
  */
+GCC_FMT_ATTR(3, 4)
 void error_propagate_prepend(Error **dst_errp, Error *local_err,
                              const char *fmt, ...);
 
-- 
2.21.3