Avoid zero-length array

While useful, this is not permitted by the standard specification.
This patch will waste a little bit of memory in affected allocations...

modules/access/rtp/session.c

@@ -149,7 +149,7 @@ struct rtp_source_t
 
     uint16_t last_seq; /* sequence of the next dequeued packet */
     block_t *blocks; /* re-ordered blocks queue */
-    void    *opaque[0]; /* Per-source private payload data */
+    void    *opaque[1]; /* Per-source private payload data */
 };
 
 /**

modules/access/sdp.c

@@ -47,7 +47,7 @@ static int Control (access_t *, int, va_list);
 struct access_sys_t
 {
     size_t length;
-    char   data[0];
+    char   data[1];
 };
 
 static int Open (vlc_object_t *obj)

modules/demux/smf.c

@@ -68,7 +68,7 @@ struct demux_sys_t
     /* by the way, "quarter note" is "noire" in French */
 
     unsigned     trackc; /* Number of tracks */
-    mtrk_t       trackv[0]; /* Track states */
+    mtrk_t       trackv[1]; /* Track states */
 };
 
 /*****************************************************************************