Fix possible null pointer warning from cppcheck

https://github.com/Hamlib/Hamlib/issues/1351

src/amplifier.c

@@ -469,7 +469,7 @@ int HAMLIB_API amp_close(AMP *amp)
     if (amp == NULL || amp->caps == NULL)
     {
         amp_debug(RIG_DEBUG_ERR, "%s: NULL ptr? amp=%p, amp->caps=%p\n", __func__, amp,
-                  amp->caps);
+                  amp->caps==NULL?NULL:amp->caps);
         return -RIG_EINVAL;
     }
 

src/multicast.c

@@ -641,7 +641,8 @@ int main(int argc, char *argv[])
 
     if (rig == NULL)
     {
-
+        fprintf(stderr, "rig==NULL?\n");
+        return 1;
     }
 
     strncpy(rig->state.rigport.pathname, "/dev/ttyUSB0", HAMLIB_FILPATHLEN - 1);