patch 9.1.0758: it's possible to set an invalid key to 'wildcharm'

Problem:  it's possible to set an invalid key to 'wildcharm'
Solution: error out, if the 'wildcharm' value is an invalid key
          (Milly)

closes: #15787

Signed-off-by: Milly <milly.ca@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/gen_opt_test.vim b/src/testdir/gen_opt_test.vim
index 936a0fa..5be06ce 100644
--- a/src/testdir/gen_opt_test.vim
+++ b/src/testdir/gen_opt_test.vim
@@ -7,6 +7,8 @@
 
 set nomore
 
+const K_KENTER = -16715
+
 " The terminal size is restored at the end.
 " Clear out t_WS, we don't want to resize the actual terminal.
 let script = [
@@ -55,7 +57,10 @@
       \ 'updatecount': [[0, 1, 8, 9999], [-1]],
       \ 'updatetime': [[0, 1, 8, 9999], [-1]],
       \ 'verbose': [[-1, 0, 1, 8, 9999], []],
-      \ 'wildcharm': [[-1, 0, 100], []],
+      \ 'wildchar': [[-1, 0, 100, 'x', '^Y', '<Esc>', '<t_xx>'],
+      \		['', 'xxx', '<xxx>', '<Esc', '<C-C>', '<NL>', '<CR>', K_KENTER]],
+      \ 'wildcharm': [[-1, 0, 100, 'x', '^Y', '<Esc>'],
+      \		['', 'xxx', '<xxx>', '<Esc', '<C-C>', '<NL>', '<CR>', K_KENTER]],
       \ 'winheight': [[1, 10, 999], [-1, 0]],
       \ 'winminheight': [[0, 1], [-1]],
       \ 'winminwidth': [[0, 1, 10], [-1]],