webvtt: fix null pointer passed in qsort

From UBSAN: null pointer passed as argument 1, which is declared to never be null

(cherry picked from commit b26cb55943)

modules/demux/webvtt.c

@@ -29,6 +29,7 @@
 #include <vlc_common.h>
 #include <vlc_demux.h>
 #include <vlc_memstream.h>
+#include <assert.h>
 
 #include "../codec/webvtt/webvtt.h"
 
@@ -325,6 +326,10 @@ static void BuildIndex( demux_t *p_demux )
 {
     demux_sys_t *p_sys = p_demux->p_sys;
 
+    if( p_sys->index.i_count == 0 )
+        return;
+    assert( p_sys->index.p_array != NULL );
+
     /* Order time entries ascending, start time before end time */
     qsort( p_sys->index.p_array, p_sys->index.i_count,
            sizeof(struct index_entry_s), index_Compare );