spkmodem-decode: ignore invalid frames when setting bits

bits are currently assembled even on invalid frames. this
patch fixes that - the bug is also in the GNU version.

this reduces the chance of noise/calibration from creating
corrupt character output during operation.

Signed-off-by: Leah Rowe <leah@libreboot.org>

util/spkmodem_decode/spkmodem-decode.c

@@ -410,10 +410,15 @@ set_ascii_bit(struct decoder_state *st)
 {
 	if (st->debug)
 		print_stats(st);
+
+	if (!is_valid_signal(st))
+		return st->ascii_bit;
+
 	if (st->freq_data < st->freq_threshold)
 		st->ascii |= (1 << st->ascii_bit);
 
 	st->ascii_bit--;
+
 	return st->ascii_bit;
 }