patch 8.0.0484: :lhelpgrep does not fail after a successful one

Problem:    Using :lhelpgrep with an argument that should fail does not
            produce an error if the previous :helpgrep worked.
Solution:   Use another way to detect that autocommands made the quickfix info
            invalid. (Yegappan Lakshmanan)
diff --git a/src/quickfix.c b/src/quickfix.c
index dbcd083..9021212 100644
--- a/src/quickfix.c
+++ b/src/quickfix.c
@@ -5077,6 +5077,7 @@
     char_u	*lang;
 #endif
     qf_info_T	*qi = &ql_info;
+    qf_info_T	*save_qi;
     int		new_qi = FALSE;
     win_T	*wp;
 #ifdef FEAT_AUTOCMD
@@ -5130,6 +5131,9 @@
 	}
     }
 
+    /* Autocommands may change the list. Save it for later comparison */
+    save_qi = qi;
+
     regmatch.regprog = vim_regcomp(eap->arg, RE_MAGIC + RE_STRING);
     regmatch.rm_ic = FALSE;
     if (regmatch.regprog != NULL)
@@ -5262,7 +5266,7 @@
     {
 	apply_autocmds(EVENT_QUICKFIXCMDPOST, au_name,
 					       curbuf->b_fname, TRUE, curbuf);
-	if (!new_qi && qi != &ql_info && qf_find_buf(qi) == NULL)
+	if (!new_qi && qi != save_qi && qf_find_buf(qi) == NULL)
 	    /* autocommands made "qi" invalid */
 	    return;
     }