Fix crash when (access_t*)->pf_control is NULL

This was triggered by DBus module using access2_Control

include/vlc_access.h

@@ -119,7 +119,7 @@ struct access_t
 
 static inline int access2_vaControl( access_t *p_access, int i_query, va_list args )
 {
-    if( !p_access ) return VLC_EGENERIC;
+    if( !p_access || !p_access->pf_control ) return VLC_EGENERIC;
     return p_access->pf_control( p_access, i_query, args );
 }