patch 9.0.1271: using sizeof() and subtract array size is tricky

Problem:    Using sizeof() and subtract array size is tricky.
Solution:   Use offsetof() instead. (closes #11926)
diff --git a/src/spell.c b/src/spell.c
index 5f83322..072740c 100644
--- a/src/spell.c
+++ b/src/spell.c
@@ -1848,7 +1848,7 @@
     hi = hash_lookup(&lp->sl_wordcount, p, hash);
     if (HASHITEM_EMPTY(hi))
     {
-	wc = alloc(sizeof(wordcount_T) + STRLEN(p));
+	wc = alloc(offsetof(wordcount_T, wc_word) + STRLEN(p) + 1);
 	if (wc == NULL)
 	    return;
 	STRCPY(wc->wc_word, p);