avutil/hwcontext_dxva2: return an error when buffer allocation fails

This also prevents the use of an uninitialized variable.
Reviewed-by: Mark Thompson <sw@jkqxz.net>
Signed-off-by: James Almer <jamrial@gmail.com>

libavutil/hwcontext_dxva2.c

@@ -307,8 +307,10 @@ static int dxva2_map_frame(AVHWFramesContext *ctx, AVFrame *dst, const AVFrame *
     }
 
     map = av_mallocz(sizeof(*map));
-    if (!map)
+    if (!map) {
+        err = AVERROR(ENOMEM);
         goto fail;
+    }
 
     err = ff_hwframe_map_create(src->hw_frames_ctx, dst, src,
                                 dxva2_unmap_frame, map);