Fix flrig modeMap loop to ignore NULL entries

https://github.com/Hamlib/Hamlib/issues/798

rigs/dummy/flrig.c

@@ -673,6 +673,7 @@ static const char *modeMapGetFLRig(rmode_t modeHamlib)
 
     for (i = 0; modeMap[i].mode_hamlib != 0; ++i)
     {
+        if (modeMap[i].mode_flrig == NULL) continue;
         rig_debug(RIG_DEBUG_TRACE,
                   "%s: checking modeMap[%d]=%.0f to modeHamlib=%.0f, mode_flrig='%s'\n", __func__,
                   i, (double)modeMap[i].mode_hamlib, (double)modeHamlib, modeMap[i].mode_flrig);