avformat/ftp: Fix Out-of-Bounds Access and Information Leak in ftp.c:393

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit a142ffdc)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/ftp.c

@@ -389,7 +389,7 @@ static int ftp_file_size(FTPContext *s)
     static const int size_codes[] = {213, 0};
 
     snprintf(command, sizeof(command), "SIZE %s\r\n", s->path);
-    if (ftp_send_command(s, command, size_codes, &res) == 213 && res) {
+    if (ftp_send_command(s, command, size_codes, &res) == 213 && res && strlen(res) > 4) {
         s->filesize = strtoll(&res[4], NULL, 10);
     } else {
         s->filesize = -1;