[Fix] Clean up Filtered/MessageString functions (#1311)

This solves the OOB issue pointed out in #1304 and cleans up the code a
bit so it should be less error prone

common/eq_packet_structs.h

@@ -3247,7 +3247,7 @@ struct TraderClick_Struct{
 };
 
 struct FormattedMessage_Struct{
-	uint32	unknown0;
+	uint32	unknown0; // 1 means from world server
 	uint32	string_id;
 	uint32	type;
 	char	message[0];
@@ -3255,7 +3255,7 @@ struct FormattedMessage_Struct{
 struct SimpleMessage_Struct{
 	uint32	string_id;
 	uint32	color;
-	uint32	unknown8;
+	uint32	unknown8; // 1 means from world server
 };
 
 struct GuildMemberUpdate_Struct {