ffmpeg: Clear error message array at init.

This avoids printing uninitialized bytes if no error message is set
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 6d1a2efb)
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffmpeg.c

@@ -2117,7 +2117,7 @@ static int transcode_init(void)
     AVCodecContext *codec;
     OutputStream *ost;
     InputStream *ist;
-    char error[1024];
+    char error[1024] = {0};
     int want_sdp = 1;
 
     for (i = 0; i < nb_filtergraphs; i++) {