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/scriptfile.c b/src/scriptfile.c
index 630a116..008c7b3 100644
--- a/src/scriptfile.c
+++ b/src/scriptfile.c
@@ -56,7 +56,7 @@
 
     // If memory allocation fails then we'll pop more than we push, eventually
     // at the top level it will be OK again.
-    if (ga_grow(&exestack, 1) != OK)
+    if (ga_grow(&exestack, 1) == FAIL)
 	return NULL;
 
     entry = ((estack_T *)exestack.ga_data) + exestack.ga_len;