qapi: Don't pass NULL to printf in string input visitor

Make sure the error message for visit_type_uint64() gracefully
handles a NULL 'name' when called from the top level or a list
context, as not all the world behaves like glibc in allowing
NULL through a printf-family %s.
Signed-off-by: NEric Blake <eblake@redhat.com>
Message-Id: <1461879932-9020-21-git-send-email-eblake@redhat.com>
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>

qapi/string-input-visitor.c

@@ -222,7 +222,7 @@ static void parse_type_int64(Visitor *v, const char *name, int64_t *obj,
     return;
 
 error:
-    error_setg(errp, QERR_INVALID_PARAMETER_VALUE, name,
+    error_setg(errp, QERR_INVALID_PARAMETER_VALUE, name ? name : "null",
                "an int64 value or range");
 }