Patch for a crash in POP when TLS errors happen

[Sergio Villar Senin <svillar igalia com>]

Hi,

this patch fixes a crash when trying to connect to a POP3 server, which
for example requires SSL, and only TLS was specified.

The problem is the following:
1- connect_to_server of camel-pop3-store.c fails and then it goes to
stls_exception.
2- then that code executes

		/* try to disconnect cleanly */
		pc = camel_pop3_engine_command_new (store->engine, 0, NULL, NULL,
"QUIT\r\n");
		while (camel_pop3_engine_iterate (store->engine, NULL) > 0)
			;
		camel_pop3_engine_command_free (store->engine, pc);

3- the problem is that this camel_pop3_engine_iterate can (and it indeed
does) issue a camel_service_disconnect which for pop calls
pop3_disconnect_online, that in line 178 does store->engine = NULL

4- so the after the "innocent" call to camel_pop3_engine_iterate we'll
get a NULL store->engine, amazing he? :-)

5- you all know what happens when we later call
camel_object_unref(store->engine) ;-)

Br

Index: libtinymail-camel/camel-lite/camel/providers/pop3/camel-pop3-store.c
===================================================================
--- libtinymail-camel/camel-lite/camel/providers/pop3/camel-pop3-store.c	(revision 3686)
+++ libtinymail-camel/camel-lite/camel/providers/pop3/camel-pop3-store.c	(working copy)
@@ -581,7 +581,10 @@
 		camel_pop3_engine_command_free (store->engine, pc);
 	}
 
-	camel_object_unref (CAMEL_OBJECT (store->engine));
+	/* camel_pop3_engine_iterate could issue a disconnect that
+	   could set engine to NULL */
+	if (store->engine)
+		camel_object_unref (CAMEL_OBJECT (store->engine));
 	camel_object_unref (CAMEL_OBJECT (tcp_stream));
 	g_static_rec_mutex_lock (store->eng_lock);
 	store->engine = NULL;