matroskaparse: don't error out if there's not enough data in the adapter
matroskaparse: don't error out if there's not enough data in the adapter

gst_matroska_parse_take() would return FLOW_ERROR instead of
FLOW_EOS in case there's less data in the adapter than requested,
because buffer is NULL in that case which triggers the error
code path. This made the unit test fail (occasionally at least,
because of a bug in the unit test there's a race and it would
happen only sporadically).

wkatsak/gst-plugins-good,jcaden/gst-plugins-good,jcaden/gst-plugins-good,GStreamer/gst-plugins-good,GStreamer/gst-plugins-good,greg80303/gst-plugins-good,chamois94/gst-plugins-good,surround-io/gst-plugins-good,ndufresne/gst-plugins-good,Lachann/gst-plugins-good,vatavuserban/gst-plugins-good,froggatt/gst-plugins-good-m,rawoul/gst-plugins-good,jpakkane/gstreamer-plugins-good,shelsonjava/gst-plugins-good,rawoul/gst-plugins-good,pexip/gst-plugins-good,freedesktop-unofficial-mirror/gstreamer__gst-plugins-good,surround-io/gst-plugins-good,sebras/gst-plugins-good,hizukiayaka/gst-plugins-good,chamois94/gst-plugins-good,pexip/gst-plugins-good,sh0/gst-plugins-good,chamois94/gst-plugins-good,GrokImageCompression/gst-plugins-good,Kurento/gst-plugins-good,hizukiayaka/gst-plugins-good,jcaden/gst-plugins-good,ijsf/OpenWebRTC-gst-plugins-good,sebras/gst-plugins-good,loshca/gst-plugins-good,sh0/gst-plugins-good,ndufresne/gst-plugins-good,loshca/gst-plugins-good,hizukiayaka/gst-plugins-good,StreamUtils/gst-plugins-good,jcaden/gst-plugins-good,Lachann/gst-plugins-good,shelsonjava/gst-plugins-good,stfl/gst-plugins-good,Lachann/gst-plugins-good,Kurento/gst-plugins-good,ijsf/OpenWebRTC-gst-plugins-good,veo-labs/gst-plugins-good,ikonst/gst-plugins-good,chamois94/gst-plugins-good,greg80303/gst-plugins-good,froggatt/gst-plugins-good-m,veo-labs/gst-plugins-good,jpakkane/gstreamer-plugins-good,stfl/gst-plugins-good,GrokImageCompression/gst-plugins-good,Kurento/gst-plugins-good,sh0/gst-plugins-good,ijsf/OpenWebRTC-gst-plugins-good,BigBrother-International/gst-plugins-good,froggatt/gst-plugins-good-m,Lachann/gst-plugins-good,stfl/gst-plugins-good,sh0/gst-plugins-good,Kurento/gst-plugins-good,GStreamer/gst-plugins-good,Kurento/gst-plugins-good,jpakkane/gstreamer-plugins-good,vatavuserban/gst-plugins-good,wkatsak/gst-plugins-good,wkatsak/gst-plugins-good,pexip/gst-plugins-good,StreamUtils/gst-plugins-good,kittee/gst-plugins-good,vatavuserban/gst-plugins-good,veo-labs/gst-plugins-good,stfl/gst-plugins-good,froggatt/gst-plugins-good-m,shelsonjava/gst-plugins-good,rawoul/gst-plugins-good,sebras/gst-plugins-good,pexip/gst-plugins-good,ikonst/gst-plugins-good,rawoul/gst-plugins-good,surround-io/gst-plugins-good,loshca/gst-plugins-good,vatavuserban/gst-plugins-good,GrokImageCompression/gst-plugins-good,wkatsak/gst-plugins-good,StreamUtils/gst-plugins-good,surround-io/gst-plugins-good,loshca/gst-plugins-good,kittee/gst-plugins-good,jpakkane/gstreamer-plugins-good,sebras/gst-plugins-good,GStreamer/gst-plugins-good,BigBrother-International/gst-plugins-good,freedesktop-unofficial-mirror/gstreamer__gst-plugins-good,pexip/gst-plugins-good,ikonst/gst-plugins-good,kittee/gst-plugins-good,freedesktop-unofficial-mirror/gstreamer__gst-plugins-good,ndufresne/gst-plugins-good,greg80303/gst-plugins-good,veo-labs/gst-plugins-good,BigBrother-International/gst-plugins-good,greg80303/gst-plugins-good,ikonst/gst-plugins-good,freedesktop-unofficial-mirror/gstreamer__gst-plugins-good,shelsonjava/gst-plugins-good,ijsf/OpenWebRTC-gst-plugins-good,GrokImageCompression/gst-plugins-good,StreamUtils/gst-plugins-good,hizukiayaka/gst-plugins-good,BigBrother-International/gst-plugins-good,ndufresne/gst-plugins-good,kittee/gst-plugins-good