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/spellfile.c b/src/spellfile.c
index 1a7bd1f..710e438 100644
--- a/src/spellfile.c
+++ b/src/spellfile.c
@@ -3428,7 +3428,7 @@
     fromto_T	*ftp;
     char_u	word[MAXWLEN];
 
-    if (ga_grow(gap, 1) != OK)
+    if (ga_grow(gap, 1) == FAIL)
 	return;
 
     ftp = ((fromto_T *)gap->ga_data) + gap->ga_len;