Fix other compiler warning

Same issue, different function.
diff --git a/c_src/b64url.c b/c_src/b64url.c
index f4067fb..5e48957 100644
--- a/c_src/b64url.c
+++ b/c_src/b64url.c
@@ -494,7 +494,7 @@
     b64url_st* st = NULL;
     size_t tlen;
     int status;
-    ENTERM ret;
+    ENTERM ret = priv->atom_error;
 
     if(argc != 1) {
         return enif_make_badarg(env);