patch 9.0.1251: checking returned value of ga_grow() is inconsistent

Problem:    Checking returned value of ga_grow() is inconsistent.
Solution:   Check for FAIL instaed of "not OK". (Yegappan Lakshmanan,
            closes #11897)
diff --git a/src/digraph.c b/src/digraph.c
index ac51d50..fb45c15 100644
--- a/src/digraph.c
+++ b/src/digraph.c
@@ -1682,7 +1682,7 @@
     }
 
     // Add a new digraph to the table.
-    if (ga_grow(&user_digraphs, 1) != OK)
+    if (ga_grow(&user_digraphs, 1) == FAIL)
 	return;
 
     dp = (digr_T *)user_digraphs.ga_data + user_digraphs.ga_len;