MINOR: map: Cleanup the initialisation of map descriptors.

The association between struct map_descriptor and its struct
map_reference must be done in the function map_create_descriptor().
diff --git a/src/map.c b/src/map.c
index 81c61eb..2ed9f3a 100644
--- a/src/map.c
+++ b/src/map.c
@@ -168,6 +168,7 @@
 		return NULL;
 
 	desc->conv = conv;
+	desc->ref = ref;
 
 	LIST_ADDQ(&ref->maps, &desc->list);
 
@@ -366,7 +367,6 @@
 		return 0;
 	}
 
-	desc->ref = ref;
 	pattern_init_expr(&desc->pat);
 
 	/* set the match method */