input_thread_mutex protection (don't destroy unless successfully created).

(part of previous commit of main.cpp)

src/sbbs3/sbbs.h

@@ -335,6 +335,7 @@ public:
 	HANDLE	input_thread;
 	pthread_mutex_t	input_thread_mutex;
 	bool	input_thread_mutex_locked;	// by someone other than the input_thread
+	bool	input_thread_mutex_created;
 	pthread_mutex_t	ssh_mutex;
 	bool	ssh_mutex_created;