avfilter/vf_thumbnail_cuda: Set ret before checking it

Fixes: CID1418336 Logically dead code

Sponsored-by: Sovereign Tech Fund
Reviewed-by: Timo Rothenpieler <timo@rothenpieler.org>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 02301017d2)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavfilter/vf_thumbnail_cuda.c

@@ -290,7 +290,7 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *frame)
             hist[i] = 4 * hist[i];
     }
 
-    CHECK_CU(cu->cuCtxPopCurrent(&dummy));
+    ret = CHECK_CU(cu->cuCtxPopCurrent(&dummy));
     if (ret < 0)
         return ret;