lavd/v4l2: do not clobber the context FD in v4l2_get_device_list()

The FD opened here is local to the loop iteration, there is no reason to
store it in the context. Since read_header() may have already been
called, this may ovewrite an existing valid FD.

libavdevice/v4l2.c

@@ -1033,16 +1033,17 @@ static int v4l2_get_device_list(AVFormatContext *ctx, AVDeviceInfoList *device_l
         return ret;
     }
     while ((entry = readdir(dir))) {
+        int fd = -1;
         char device_name[256];
 
         if (!v4l2_is_v4l_dev(entry->d_name))
             continue;
 
         snprintf(device_name, sizeof(device_name), "/dev/%s", entry->d_name);
-        if ((s->fd = device_open(ctx, device_name)) < 0)
+        if ((fd = device_open(ctx, device_name)) < 0)
             continue;
 
-        if (v4l2_ioctl(s->fd, VIDIOC_QUERYCAP, &cap) < 0) {
+        if (v4l2_ioctl(fd, VIDIOC_QUERYCAP, &cap) < 0) {
             ret = AVERROR(errno);
             av_log(ctx, AV_LOG_ERROR, "ioctl(VIDIOC_QUERYCAP): %s\n", av_err2str(ret));
             goto fail;
@@ -1064,8 +1065,7 @@ static int v4l2_get_device_list(AVFormatContext *ctx, AVDeviceInfoList *device_l
                                           &device_list->nb_devices, device)) < 0)
             goto fail;
 
-        v4l2_close(s->fd);
-        s->fd = -1;
+        v4l2_close(fd);
         continue;
 
       fail:
@@ -1074,9 +1074,7 @@ static int v4l2_get_device_list(AVFormatContext *ctx, AVDeviceInfoList *device_l
             av_freep(&device->device_description);
             av_freep(&device);
         }
-        if (s->fd >= 0)
-            v4l2_close(s->fd);
-        s->fd = -1;
+        v4l2_close(fd);
         break;
     }
     closedir(dir);