centralize imap_cmd_refcounted_state refcount decrementing

no else branches remain, so the if() can be put into
imap_refcounted_done()

src/drv_imap.c

@@ -428,9 +428,11 @@ imap_refcounted_new_cmd( struct imap_cmd_refcounted_state *sts )
 static void
 imap_refcounted_done( struct imap_cmd_refcounted_state *sts )
 {
+	if (!--sts->ref_count) {
 		sts->callback( sts->ret_val, sts->callback_aux );
 		free( sts );
 	}
+}
 
 static int
 is_atom( list_t *list )
@@ -1497,7 +1499,6 @@ imap_load( store_t *gctx, int minuid, int maxuid, int *excs, int nexcs,
 		}
 	  done:
 		free( excs );
-		if (!--sts->ref_count)
 		imap_refcounted_done( sts );
 	}
 }
@@ -1525,7 +1526,6 @@ imap_load_p2( imap_store_t *ctx ATTR_UNUSED, struct imap_cmd *cmd, int response
 			sts->ret_val = DRV_BOX_BAD;
 		break;
 	}
-	if (!--sts->ref_count)
 	imap_refcounted_done( sts );
 }
 
@@ -1595,7 +1595,6 @@ imap_set_flags( store_t *gctx, message_t *msg, int uid, int add, int del,
 		struct imap_cmd_refcounted_state *sts = imap_refcounted_new_state( cb, aux );
 		if ((add && imap_flags_helper( ctx, uid, '+', add, sts ) < 0) ||
 		    (del && imap_flags_helper( ctx, uid, '-', del, sts ) < 0)) {}
-		if (!--sts->ref_count)
 		imap_refcounted_done( sts );
 	} else {
 		cb( DRV_OK, aux );
@@ -1615,7 +1614,6 @@ imap_set_flags_p2( imap_store_t *ctx ATTR_UNUSED, struct imap_cmd *cmd, int resp
 			sts->ret_val = DRV_MSG_BAD;
 		break;
 	}
-	if (!--sts->ref_count)
 	imap_refcounted_done( sts );
 }