lavd/lavfi: add check in case pix_fmts cannot be allocated in lavfi_read_header()

(cherry picked from commit 4d475f7eb3)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavdevice/lavfi.c

@@ -88,6 +88,9 @@ av_cold static int lavfi_read_header(AVFormatContext *avctx,
 
 #define FAIL(ERR) { ret = ERR; goto end; }
 
+    if (!pix_fmts)
+        FAIL(AVERROR(ENOMEM));
+
     avfilter_register_all();
 
     buffersink = avfilter_get_by_name("buffersink");