Tweak parsing ais persons as structured data

- Type 8, DAC = 1 FID = 16: This should only be decoded as "Persons on board" if the message length is either 72 or 136 bits long (if even then).
- Type 8, DAC = 1 FID = 16: If decoded, the json output should use the ais->type8.dac1fid16.persons field and not the type6 field

TODO: Missing any sample data that exercises this code path - jmls

Identified by: Stefan Roels <sroels-gpsd-dev@42solutions.nl>

driver_ais.c

@@ -528,10 +528,15 @@ bool ais_binary_decode(const struct gpsd_errout_t *errout,
 		break;
 	    case 16:	    /* Number of Persons On Board */
 		if (ais->type8.bitcount == 136)
-		    ais->type8.dac1fid16.persons = UBITS(88, 13);/* 289 */
-		else
-		    ais->type8.dac1fid16.persons = UBITS(55, 13);/* 236 */
-		ais->type8.structured = true;
+		{
+			ais->type8.dac1fid16.persons = UBITS(88, 13);/* 289 */
+			ais->type8.structured = true;
+		}
+		else if (ais->type8.bitcount == 72)
+		{
+			ais->type8.dac1fid16.persons = UBITS(55, 13);/* 236 */
+			ais->type8.structured = true;
+		}
 		break;
 	    case 17:        /* IMO289 - VTS-generated/synthetic targets */
 #define ARRAY_BASE 56