Fix possible improper NULL string termination

Make sure that the language string is always terminated by a NULL
character.  strncpy will copy at most 4 characters, but no NULL
character is implicitly appended so language will only be NULL
terminated if the string has fewer than 4 characers.

xbmc/cores/dvdplayer/DVDDemuxers/DVDDemuxHTSP.cpp

@@ -325,7 +325,10 @@ void CDVDDemuxHTSP::SubscriptionStart (htsmsg_t *m)
     }
 
     if((lang = htsmsg_get_str(sub, "language")))
+    {
       strncpy(st.g->language, lang, sizeof(st.g->language));
+      st.g->language[sizeof(st.g->language) - 1] = '\0';
+    }
 
     st.g->iId         = m_Streams.size();
     st.g->iPhysicalId = index;