MDEV-20525: Fix the -std=c90 builds

Author: dr-m

dbug/dbug.c

@@ -2262,8 +2262,8 @@ extern my_bool my_assert;
 ATTRIBUTE_COLD
 my_bool _db_my_assert(const char *file, int line, const char *msg)
 {
-  _db_flush_();
   my_bool a = my_assert;
+  _db_flush_();
   if (!a)
     fprintf(stderr, "%s:%d: assert: %s\n", file, line, msg);
   return a;