logger: don't use uint8_t buffer[msg_size]; (it's C99 not C++)

Also, it's not clear where the allocation was. It looks like it was on
the heap, but the compiler could decide to put it on the stack. This is
very bad for us because we use fixed size stacks with tights bounds. So if
a user specifies a large topic to log, it could have crashed.

Now the allocation is on the heap and the user can specify any size of
topic to log (as long as there is enough memory).

src/modules/logger/logger.h

@@ -195,6 +195,8 @@ private:
 	static constexpr const char 	*LOG_ROOT = PX4_ROOTFSDIR"/fs/microsd/log";
 #endif
 
+	uint8_t						*_msg_buffer = nullptr;
+	int						_msg_buffer_len = 0;
 	bool						_task_should_exit = true;
 	char 						_log_dir[64];
 	bool						_has_log_dir = false;