AP_RCProtocol: fixed DSM parser for 8 channels

this ensures we remember channels from the previous frame, avoiding a
bug with more than 7 channels

libraries/AP_RCProtocol/AP_RCProtocol_DSM.cpp

@@ -381,11 +381,9 @@ void AP_RCProtocol_DSM::_process_byte(uint32_t timestamp_us, uint8_t b)
     byte_input.last_byte_us = timestamp_us;
     byte_input.buf[byte_input.ofs++] = b;
     if (byte_input.ofs == 16) {
-        uint16_t values[8];
-        uint16_t num_values=0;
-        if (dsm_decode(timestamp_us, byte_input.buf, values, &num_values, 8) &&
-            num_values >= MIN_RCIN_CHANNELS) {
-            add_input(num_values, values, false);
+        if (dsm_decode(timestamp_us, byte_input.buf, &last_values[0], &num_channels, AP_DSM_MAX_CHANNELS) &&
+            num_channels >= MIN_RCIN_CHANNELS) {
+            add_input(num_channels, last_values, false);
         }
 
         byte_input.ofs = 0;

libraries/AP_RCProtocol/AP_RCProtocol_DSM.h

@@ -20,6 +20,8 @@
 #include "AP_RCProtocol.h"
 #include "SoftSerial.h"
 
+#define AP_DSM_MAX_CHANNELS 12
+
 class AP_RCProtocol_DSM : public AP_RCProtocol_Backend {
 public:
     AP_RCProtocol_DSM(AP_RCProtocol &_frontend) : AP_RCProtocol_Backend(_frontend) {}
@@ -59,6 +61,9 @@ private:
     } bind_state;
     uint32_t bind_last_ms;
 
+    uint16_t last_values[AP_DSM_MAX_CHANNELS];
+    uint16_t num_channels;
+
     struct {
         uint8_t buf[16];
         uint8_t ofs;