sip: provider: Keep provider alive while there are still origins

CallsSipOrigin accesses the CallsSipContext (owned by the provider) in its
dispose() which will be invalid once CallsSipProvider gets freed.
This commit will make certain that the provider stays alive for the lifetime
of it's origins.

plugins/sip/calls-sip-provider.c

@@ -363,6 +363,8 @@ add_account (CallsAccountProvider *acc_provider,
 
   self = CALLS_SIP_PROVIDER (acc_provider);
 
+  g_object_ref (acc_provider);
+
   return !!calls_sip_provider_add_origin (self, credentials, 0, FALSE);
 }
 
@@ -391,6 +393,8 @@ remove_account (CallsAccountProvider *acc_provider,
   g_hash_table_remove (self->credentials, credentials);
   g_list_store_remove (self->origins, position);
 
+  g_object_unref (acc_provider);
+
   return TRUE;
 }