avdevice/dshow: Check device_filter_unique_name before use

Fixes: CID1591931 Explicit null dereferenced

Sponsored-by: Sovereign Tech Fund
Reviewed-by: Roger Pack <rogerdpack@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 175c191668)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavdevice/dshow.c

@@ -1372,7 +1372,7 @@ dshow_open_device(AVFormatContext *avctx, ICreateDevEnum *devenum,
     if (ctx->device_filter[otherDevType]) {
         // avoid adding add two instances of the same device to the graph, one for video, one for audio
         // a few devices don't support this (could also do this check earlier to avoid double crossbars, etc. but they seem OK)
-        if (strcmp(device_filter_unique_name, ctx->device_unique_name[otherDevType]) == 0) {
+        if (!device_filter_unique_name || strcmp(device_filter_unique_name, ctx->device_unique_name[otherDevType]) == 0) {
           av_log(avctx, AV_LOG_DEBUG, "reusing previous graph capture filter... %s\n", device_filter_unique_name);
           IBaseFilter_Release(device_filter);
           device_filter = ctx->device_filter[otherDevType];