visual: check for NULL before free

Close #6879
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>
(cherry picked from commit 73e24627)
Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>

modules/visualization/visual/visual.c

@@ -373,20 +373,23 @@ static void Close( vlc_object_t *p_this )
     for( int i = 0; i < p_sys->i_effect; i++ )
     {
 #define p_effect p_sys->effect[i]
+        if( p_effect->p_data != NULL )
+        {
             if( !strncmp( p_effect->psz_name, "spectrum", strlen( "spectrum" ) ) )
             {
-            spectrum_data *p_data = p_effect->p_data;
+                spectrum_data* p_data = p_effect->p_data;
                 free( p_data->peaks );
                 free( p_data->prev_heights );
                 free( p_data->p_prev_s16_buff );
             }
             if( !strncmp( p_effect->psz_name, "spectrometer", strlen( "spectrometer" ) ) )
             {
-            spectrometer_data *p_data = p_effect->p_data;
+                spectrometer_data* p_data = p_effect->p_data;
                 free( p_data->peaks );
                 free( p_data->p_prev_s16_buff );
             }
             free( p_effect->p_data );
+        }
         free( p_effect->psz_args );
         free( p_effect );
 #undef p_effect