patch 8.0.1499: out-of-memory situation not correctly handled

Problem:    Out-of-memory situation not correctly handled. (Coverity)
Solution:   Check for NULL value.
diff --git a/src/terminal.c b/src/terminal.c
index b67cd6c..6b8d353 100644
--- a/src/terminal.c
+++ b/src/terminal.c
@@ -3153,6 +3153,8 @@
 	    bg = cell.bg;
 	}
 	dcell = dict_alloc();
+	if (dcell == NULL)
+	    break;
 	list_append_dict(l, dcell);
 
 	dict_add_nr_str(dcell, "chars", 0, mbs);