patch 8.2.3669: buffer overflow with long help argument

Problem:    Buffer overflow with long help argument.
Solution:   Use snprintf().

src/help.c

@@ -422,8 +422,7 @@ find_help_tags(
 		    || (vim_strchr((char_u *)"%_z@", arg[1]) != NULL
 							   && arg[2] != NUL)))
 	{
-	    STRCPY(d, "/\\\\");
+	    vim_snprintf((char *)d, IOSIZE, "/\\\\%s", arg + 1);
-	    STRCPY(d + 3, arg + 1);
 	    // Check for "/\\_$", should be "/\\_\$"
 	    if (d[3] == '_' && d[4] == '$')
 		STRCPY(d + 4, "\\$");

src/testdir/test_help.vim

@@ -134,4 +134,13 @@ func Test_help_window_height()
   close
 endfunc
 
+func Test_help_long_argument()
+  try
+    exe 'help \%' .. repeat('0', 1021)
+  catch
+    call assert_match("E149:", v:exception)
+  endtry
+endfunc
+
+
 " vim: shiftwidth=2 sts=2 expandtab

src/version.c

@@ -757,6 +757,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3669,
 /**/
     3668,
 /**/