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/usercmd.c b/src/usercmd.c
index 9dcc20e..44fa578 100644
--- a/src/usercmd.c
+++ b/src/usercmd.c
@@ -1058,7 +1058,7 @@
     // Extend the array unless we're replacing an existing command
     if (cmp != 0)
     {
-	if (ga_grow(gap, 1) != OK)
+	if (ga_grow(gap, 1) == FAIL)
 	    goto fail;
 	if ((p = vim_strnsave(name, name_len)) == NULL)
 	    goto fail;