AP_RCProtocol: increased robustness of SBUS parsing

take full advantage of the frame gap to reduce CPU and the chance of a
bad decode

libraries/AP_RCProtocol/AP_RCProtocol_SBUS.cpp

@@ -226,11 +226,22 @@ void AP_RCProtocol_SBUS::process_pulse(uint32_t width_s0, uint32_t width_s1)
 // support byte input
 void AP_RCProtocol_SBUS::_process_byte(uint32_t timestamp_us, uint8_t b)
 {
-    if (timestamp_us - byte_input.last_byte_us > 2000U ||
+    const bool have_frame_gap = (timestamp_us - byte_input.last_byte_us > 2000U);
+    if (have_frame_gap &&
         byte_input.ofs == sizeof(byte_input.buf)) {
         byte_input.ofs = 0;
     }
     byte_input.last_byte_us = timestamp_us;
+
+    if (b != 0x0F && byte_input.ofs == 0) {
+        // definately not SBUS, missing header byte
+        return;
+    }
+    if (byte_input.ofs == 0 && !have_frame_gap) {
+        // must have a frame gap before the start of a new SBUS frame
+        return;
+    }
+
     byte_input.buf[byte_input.ofs++] = b;
 
     if (byte_input.ofs == sizeof(byte_input.buf)) {