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/evalfunc.c b/src/evalfunc.c
index 7b34e39..08f8847 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -3966,7 +3966,7 @@
 	len = (int)STRLEN(value);	// Append the entire string
     else
 	len = value_len;		// Append only "value_len" characters
-    if (ga_grow(&redir_execute_ga, len) != OK)
+    if (ga_grow(&redir_execute_ga, len) == FAIL)
 	return;
 
     mch_memmove((char *)redir_execute_ga.ga_data