lavc/qsv_overlay: suppress code scan complain

Suppress the complain "variables 'pix_fmt' is used but maybe
uninitialized".
Signed-off-by: Zhong Li <zhong.li@intel.com>

libavfilter/vf_overlay_qsv.c

@@ -160,7 +160,7 @@ release:
 
 static int have_alpha_planar(AVFilterLink *link)
 {
-    enum AVPixelFormat pix_fmt;
+    enum AVPixelFormat pix_fmt = link->format;
     const AVPixFmtDescriptor *desc;
     AVHWFramesContext *fctx;