Check whether serial_serno is not NULL before calling it.

For devices connected through a real serial device rather than
USB, there is no transport-layer serial number available.

stk500v2.c (and jtag3.c) tried to always call this function.

Closes #1254

NEWS

@@ -122,6 +122,7 @@ Changes since version 7.0:
     - megaAVR JTAG programming doesn't work for SNAP and PICkit4
       #1052
     - urclock/FTDI communication issues #1245
+    - stk500v2.c dumps core on open #1254
 
   * Pull requests:
 

src/jtag3.c

@@ -1570,6 +1570,7 @@ int jtag3_open_common(PROGRAMMER *pgm, const char *port) {
   }
 
   // Get USB serial number
+  if (serial_serno)
     pgm->usbsn = serial_serno();
 
   /*

src/stk500v2.c

@@ -1579,6 +1579,7 @@ static int stk500v2_open(PROGRAMMER *pgm, const char *port) {
   }
 
   // Get USB serial number
+  if (serial_serno)
     pgm->usbsn = serial_serno();
 
   /*