af_channelmap: fix ONE_STR mapping mode

get_channel() returns 0 on success

CC:libav-stable@libav.org
(cherry picked from commit e8436126)

libavfilter/af_channelmap.c

@@ -188,7 +188,7 @@ static av_cold int channelmap_init(AVFilterContext *ctx)
             s->map[i].out_channel_idx = i;
             break;
         case MAP_ONE_STR:
-            if (!get_channel(&mapping, &in_ch, separator)) {
+            if (get_channel(&mapping, &in_ch, separator) < 0) {
                 av_log(ctx, AV_LOG_ERROR, err);
                 return AVERROR(EINVAL);
             }