applehttp: fix variant discard logic

The v->ctx is always not NULL now, check for streams presence to
mark the read_header state.

Fixes bug #25, possibly introduced by 603b8bc

libavformat/applehttp.c

@@ -412,7 +412,7 @@ reload:
     c->end_of_segment = 1;
     c->cur_seq_no = v->cur_seq_no;
 
-    if (v->ctx) {
+    if (v->ctx && v->ctx->nb_streams) {
         v->needed = 0;
         for (i = v->stream_offset; i < v->stream_offset + v->ctx->nb_streams;
              i++) {