ffmpeg: Check read_ffserver_streams() return value

The function avformat_alloc_context() will return a NULL pointer on
failure. However, in function read_ffserver_streams(), its return value
is not validated and the subsequent dereference may result in a bad
memory access bug. Check its return value against NULL and avoid
potential NULL dereference.
Signed-off-by: Pan Bian <bianpan2016@163.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

fftools/ffmpeg_opt.c

@@ -2000,6 +2000,8 @@ static int read_ffserver_streams(OptionsContext *o, AVFormatContext *s, const ch
 {
     int i, err;
     AVFormatContext *ic = avformat_alloc_context();
+    if (!ic)
+        return AVERROR(ENOMEM);
 
     ic->interrupt_callback = int_cb;
     err = avformat_open_input(&ic, filename, NULL, NULL);