Fix a state machine defect with repeated PTT reset calls

When the PTT serial  port is not the control port  the intention is to
free the port  when PTT is reset so that  other applications can share
it. This logic  was faulty with attempts to call  port functions on an
unopened port when repeatedly calling the rig_set_ptt API.

Thanks to Christoph van Wullen, DL1YCF, for finding and analyzing this
defect.

src/rig.c

@@ -1333,9 +1333,14 @@ int HAMLIB_API rig_set_ptt(RIG *rig, vfo_t vfo, ptt_t ptt)
 		break;
 
 	case RIG_PTT_SERIAL_DTR:
+		/* when the PTT port is not the control port we want to free the
+			 port when PTT is reset and seize the port when PTT is set,
+			 this allows limited sharing of the PTT port between
+			 applications so long as there is no contention */
 	  if (strcmp(rs->pttport.pathname, rs->rigport.pathname)
-				&& ptt!=RIG_PTT_OFF && rs->pttport.fd < 0)
+				&& rs->pttport.fd < 0)
 	    {
+				if (RIG_PTT_OFF == ptt) return RIG_OK; /* nothing to do here */
 	      rs->pttport.fd = ser_open(&rs->pttport);
 	      if (rs->pttport.fd < 0)
           {
@@ -1359,9 +1364,14 @@ int HAMLIB_API rig_set_ptt(RIG *rig, vfo_t vfo, ptt_t ptt)
 		return retcode;
 
 	case RIG_PTT_SERIAL_RTS:
+		/* when the PTT port is not the control port we want to free the
+			 port when PTT is reset and seize the port when PTT is set,
+			 this allows limited sharing of the PTT port between
+			 applications so long as there is no contention */
 	  if (strcmp(rs->pttport.pathname, rs->rigport.pathname)
-				&& ptt!=RIG_PTT_OFF && rs->pttport.fd < 0)
+				&& rs->pttport.fd < 0)
 	    {
+				if (RIG_PTT_OFF == ptt) return RIG_OK; /* nothing to do here */
 	      rs->pttport.fd = ser_open(&rs->pttport);
 	      if (rs->pttport.fd < 0)
           {