Fix byte-stuffing issues (#32)

This commit fixes two issues related to byte stuffing in SHDLC

1) Decoding SHDLC messages fail when the CRC byte is stuffed.
   This issue was reported upstream, along with a fix. Many thanks.

2) Missing break; in case statement causing an unwanted fall-through.
   Stuffing bytes causes the buffer to not only add the stuffing
   marker and the stuffed byte, but also the original unstuffed byte.
   This means that the data is both too long and corrupt, likely
   causing the sensor to ignore the request.

embedded-uart-common/sensirion_shdlc.c

@@ -70,6 +70,7 @@ static u16 sensirion_shdlc_stuff_data(u8 data_len, const u8 *data,
                 *(stuffed_data++) = 0x7d;
                 *(stuffed_data++) = c ^ (1 << 5);
                 output_data_len += 2;
+                break;
             default:
                 *(stuffed_data++) = c;
                 output_data_len += 1;
@@ -185,7 +186,7 @@ s16 sensirion_shdlc_rx(u8 max_data_len, struct sensirion_shdlc_rx_header *rxh,
 
     crc = rx_frame[i++];
     if (sensirion_shdlc_check_unstuff(crc)) {
-        crc = sensirion_shdlc_unstuff_byte(rx_frame[++i]);
+        crc = sensirion_shdlc_unstuff_byte(rx_frame[i++]);
         if (i >= len)
             return SENSIRION_SHDLC_ERR_MISSING_STOP;
     }