image: check decoder validity before clearing fmt_in

The input format must then only be cleared when a decoder is present.
Decoder not being set is a perfectly valid scenario when only generating
an image.

This patch address a crash introduced by 08b2486

src/misc/image.c

@@ -117,9 +117,12 @@ void image_HandlerDelete( image_handler_t *p_image )
 {
     if( !p_image ) return;
 
-    struct decoder_owner *p_owner = dec_get_owner( p_image->p_dec );
-    es_format_Clean( &p_owner->fmt_in );
-    decoder_Destroy( p_image->p_dec );
+    if( p_image->p_dec != NULL )
+    {
+        struct decoder_owner *p_owner = dec_get_owner( p_image->p_dec );
+        es_format_Clean( &p_owner->fmt_in );
+        decoder_Destroy( p_image->p_dec );
+    }
     if( p_image->p_enc )
         vlc_encoder_Destroy( p_image->p_enc );
     if( p_image->p_converter ) DeleteConverter( p_image->p_converter );