blkdebug: Allow usage without config file

As soon as new rules can be set during runtime, as introduced by the
next patch, blkdebug makes sense even without a config file.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/blkdebug.c

@@ -240,6 +240,11 @@ static int read_config(BDRVBlkdebugState *s, const char *filename)
     int ret;
     struct add_rule_data d;
 
+    /* Allow usage without config file */
+    if (!*filename) {
+        return 0;
+    }
+
     f = fopen(filename, "r");
     if (f == NULL) {
         return -errno;