lavfi/vf_overlay: fix unitialized pointers

Missed in commit 301c2784b3.

Found-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>

libavfilter/vf_overlay.c

@@ -245,8 +245,8 @@ static int query_formats(AVFilterContext *ctx)
         AV_PIX_FMT_NONE
     };
 
-    AVFilterFormats *main_formats;
-    AVFilterFormats *overlay_formats;
+    AVFilterFormats *main_formats = NULL;
+    AVFilterFormats *overlay_formats = NULL;
     int ret;
 
     switch (s->format) {