mjpeg: do not assume that stream_Read() succeeds

(cherry picked from commit f03f44d76a1949f5cbc05989fa1ab257818d245c)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/demux/mjpeg.c

@@ -462,7 +462,8 @@ static int MimeDemux( demux_t *p_demux )
 
     if( i_size > 0 )
     {
-        stream_Read( p_demux->s, NULL, i_size );
+        if( stream_Read( p_demux->s, NULL, i_size ) != i_size )
+            return 0;
     }
     else if( i_size < 0 )
     {