fix Shearwater PNF parsing for Petrel / Petrel 2

At some point (possibly around v71 of their firmware), Shearwater implemented
PNF for the Petrel and Petrel 2. Those are of course not air integrated, and
apparently don't support adjustable sample rate, so the log data doesn't include
opening and closing record 5. Instead of failing when those aren't found, we
should simply only access those when they actually exist.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

src/shearwater_predator_parser.c

@@ -562,7 +562,10 @@ shearwater_predator_parser_cache (shearwater_predator_parser_t *parser)
 	}
 
 	// Verify the required opening/closing records.
-	for (unsigned int i = 0; i < NRECORDS - 2; ++i) {
+	// At least in firmware v71 and newer, Petrel and Petrel 2 also use PNF,
+	// and there opening/closing record 5 (which contains AI information plus
+	// the sample interval) don't appear to exist - so don't mark them as required
+	for (unsigned int i = 0; i <= 4; ++i) {
 		if (parser->opening[i] == UNDEFINED || parser->closing[i] == UNDEFINED) {
 			ERROR (abstract->context, "Opening or closing record %u not found.", i);
 			return DC_STATUS_DATAFORMAT;
@@ -739,7 +742,7 @@ shearwater_predator_parser_samples_foreach (dc_parser_t *abstract, dc_sample_cal
 	// Sample interval.
 	unsigned int time = 0;
 	unsigned int interval = 10;
-	if (parser->pnf && parser->logversion >= 9) {
+	if (parser->pnf && parser->logversion >= 9 && parser->opening[5] != UNDEFINED) {
 		interval = array_uint16_be (data + parser->opening[5] + 23);
 		if (interval % 1000 != 0) {
 			ERROR (abstract->context, "Unsupported sample interval (%u ms).", interval);