[calls] call: Prefer using enum values

[Evangelos Ribeiro Tzaras <devrtz src gnome org>]

commit 1d20998e9a9e51062dc666359fe04ec2ada84725
Author: Evangelos Ribeiro Tzaras <devrtz fortysixandtwo eu>
Date:   Thu Jun 23 00:59:02 2022 +0200

    call: Prefer using enum values

 src/calls-call.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/calls-call.c b/src/calls-call.c
index 2d9d069a..4b0fa1a3 100644
--- a/src/calls-call.c
+++ b/src/calls-call.c
@@ -430,7 +430,7 @@ calls_call_get_state (CallsCall *self)
 {
   CallsCallPrivate *priv = calls_call_get_instance_private (self);
 
-  g_return_val_if_fail (CALLS_IS_CALL (self), 0);
+  g_return_val_if_fail (CALLS_IS_CALL (self), CALLS_CALL_STATE_UNKNOWN);
 
   return priv->state;
 }