MAVLink: Limit use to 3 instances, which is what is realistically being used.

src/modules/mavlink/mavlink_main.cpp

@@ -1789,6 +1789,12 @@ Mavlink::start(int argc, char *argv[])
 	// before returning to the shell
 	int ic = Mavlink::instance_count();
 
+	if (ic == MAVLINK_COMM_NUM_BUFFERS) {
+		warnx("Maximum MAVLink instance count of %d reached.",
+			(int)MAVLINK_COMM_NUM_BUFFERS);
+		return 1;
+	}
+
 	// Instantiate thread
 	char buf[24];
 	sprintf(buf, "mavlink_if%d", ic);

src/modules/mavlink/module.mk

@@ -53,6 +53,6 @@ MAXOPTIMIZATION	 = -Os
 
 MODULE_STACKSIZE = 1200
 
-EXTRACXXFLAGS	= -Weffc++ -Wno-attributes -Wno-packed
+EXTRACXXFLAGS	= -Weffc++ -Wno-attributes -Wno-packed -DMAVLINK_COMM_NUM_BUFFERS=3
 
-EXTRACFLAGS	= -Wno-packed
+EXTRACFLAGS	= -Wno-packed -DMAVLINK_COMM_NUM_BUFFERS=3