lavf/matroskaenc: Force the minimum value for -reserve_index_space to 2.

Fixes an assertion failure:
Assertion size >= 2 failed at libavformat/matroskaenc.c:298

libavformat/matroskaenc.c

@@ -2001,6 +2001,8 @@ static int mkv_write_header(AVFormatContext *s)
     }
     if ((pb->seekable & AVIO_SEEKABLE_NORMAL) && mkv->reserve_cues_space) {
         mkv->cues_pos = avio_tell(pb);
+        if (mkv->reserve_cues_space == 1)
+            mkv->reserve_cues_space++;
         put_ebml_void(pb, mkv->reserve_cues_space);
     }