Protect against bad use after a vlc_dictionary_clear.

(cherry picked from commit 2909c5c3)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

include/vlc_arrays.h

@@ -442,6 +442,7 @@ static inline void vlc_dictionary_clear( vlc_dictionary_t * p_dict )
             }
         }
         free( p_dict->p_entries );
+        p_dict->p_entries = NULL;
     }
     p_dict->i_size = 0;
 }