Fix sprintflst mode to skip empty modes

src/sprintflst.c

@@ -95,12 +95,12 @@ int rig_sprintf_mode(char *str, int nlen, rmode_t mode)
     for (i = 0; i < HAMLIB_MAX_MODES; i++)
     {
         const char *ms = rig_strrmode(mode & (1ULL << i));
-        if (i > 0) strcat(str, " ");
 
         if (!ms || !ms[0])
         {
             continue;    /* unknown, FIXME! */
         }
+        if (i > 0) strcat(str, " ");
 
         strcat(str, ms);
         len += strlen(ms) + 1;