[gnumeric] graph: protect against critical.

[Morten Welinder <mortenw src gnome org>]

commit 387d1112eb9764516eb11019a116c06dc18f399f
Author: Morten Welinder <terra gnome org>
Date:   Sun Apr 19 09:00:34 2020 -0400

    graph: protect against critical.
    
    It isn't clear why this occurs, but preventing the critical is easy.
    Fallout from #473.

 src/graph.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/graph.c b/src/graph.c
index c68a4c387..59a8fce23 100644
--- a/src/graph.c
+++ b/src/graph.c
@@ -1190,7 +1190,7 @@ gnm_go_data_matrix_load_size (GODataMatrix *dat)
        int old_rows = dat->size.rows, old_columns = dat->size.columns;
 
        eval_pos_init_dep (&ep, &mat->dep);
-       if (mat->val == NULL) {
+       if (mat->val == NULL && mat->dep.texpr) {
                mat->val = gnm_expr_top_eval (mat->dep.texpr, &ep,
                        GNM_EXPR_EVAL_PERMIT_NON_SCALAR | GNM_EXPR_EVAL_PERMIT_EMPTY);
        }