coroutine: add NOT_NULL_OR_DIE() convenience macro

Returns its argument if it's not a NULL pointer, otherwise cancels the
running task.

src/coroutine.h

@@ -496,6 +496,15 @@ DECLARE_EXTERN_TASK(_cancel_task_helper, { BoxedTask task; });
 #define STALL                cotask_wait(INT_MAX)
 #define AWAIT_SUBTASKS       cotask_wait_subtasks()
 
+#define NOT_NULL_OR_DIE(expr) ({ \
+	__auto_type _not_null_ptr = (expr); \
+	if(_not_null_ptr == NULL) { \
+		cotask_cancel(NOT_NULL(cotask_active())); \
+		UNREACHABLE; \
+	} \
+	NOT_NULL(_not_null_ptr); \
+})
+
 // first arg of the generated function needs to be the ent, because ENT_UNBOXED_DISPATCH_FUNCTION dispatches on first arg.
 #define _cotask_emit_bindfunc(typename, ...) \
 	INLINE typename *_cotask_bind_to_entity_##typename(typename *ent, CoTask *task) { \