vim-patch:9.1.2133: Another case of buffer overflow with 'helpfile' (#37746)

Problem:  Another case of buffer overflow with 'helpfile'.
Solution: Leave room for "tags" in the buffer (zeertzjq).

closes: vim/vim#19340

21d591fb12
(cherry picked from commit 15061d322d)

src/nvim/tag.c

@@ -2495,7 +2495,7 @@ int get_tagfname(tagname_T *tnp, int first, char *buf)
         return FAIL;
       }
       tnp->tn_hf_idx++;
-      xstrlcpy(buf, p_hf, MAXPATHL);
+      xstrlcpy(buf, p_hf, MAXPATHL - STRLEN_LITERAL("tags"));
       STRCPY(path_tail(buf), "tags");
 #ifdef BACKSLASH_IN_FILENAME
       slash_adjust(buf);

test/old/testdir/test_help.vim

@@ -238,6 +238,11 @@ func Test_helpfile_overflow()
   let &helpfile = repeat('A', 5000)
   help
   helpclose
+  for i in range(4089, 4096)
+    let &helpfile = repeat('A', i) .. '/A'
+    help
+    helpclose
+  endfor
   let &helpfile = _helpfile
 endfunc