patch 8.2.4668: buffer allocation failures insufficiently tested

Problem:    Buffer allocation failures insufficiently tested.
Solution:   Add tests for memory allocation failures. (Yegappan Lakshmanan,
            closes #10064)
diff --git a/src/buffer.c b/src/buffer.c
index 93da5dc..c8e63c3 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -2093,7 +2093,7 @@
 	}
 #ifdef FEAT_EVAL
 	// init b: variables
-	buf->b_vars = dict_alloc();
+	buf->b_vars = dict_alloc_id(aid_buflistnew_bvars);
 	if (buf->b_vars == NULL)
 	{
 	    vim_free(ffname);