only print error when interface intialization failed not when it succeeded.

(cherry picked from commit 503c860af9e3152af19d7123fae37727c82896b7)
Signed-off-by: Derk-Jan Hartman <hartman@videolan.org>

src/control/core.c

@@ -187,7 +187,7 @@ void libvlc_release( libvlc_instance_t *p_instance )
 void libvlc_add_intf( libvlc_instance_t *p_i, const char *name,
                       libvlc_exception_t *p_e )
 {
-    if( libvlc_InternalAddIntf( p_i->p_libvlc_int, name ) )
+    if( !libvlc_InternalAddIntf( p_i->p_libvlc_int, name ) )
         RAISEVOID( "Interface initialization failed" );
 }