fix ":menu Item.SubItem"

:menu should print sub-menu contents. E.g. this should print the
"File.Save" submenu:

    nvim -u NORC
    :source $VIMRUNTIME/menu.vim
    :menu File.Save

Regressed in dc685387a3
Blocks #8173

menu_get() also was missing some results for some cases.

src/nvim/menu.c

@@ -754,23 +754,28 @@ bool menu_get(char_u *const path_name, int modes, list_T *list)
 /// @param menu top menu to start looking from
 /// @param name path towards the menu
 /// @return menu if \p name is null, found menu or NULL
-vimmenu_T *
-find_menu(vimmenu_T *menu, char_u * name, int modes)
+static vimmenu_T* find_menu(vimmenu_T *menu, char_u * name, int modes)
 {
   char_u *p;
 
   while (*name) {
+    // find the end of one dot-separated name and put a NUL at the dot
     p = menu_name_skip(name);
     while (menu != NULL) {
       if (menu_name_equal(name, menu)) {
-        /* Found menu */
+        // Found menu
         if (*p != NUL && menu->children == NULL) {
+          if (*p != NUL) {
             EMSG(_(e_notsubmenu));
             return NULL;
           } else if ((menu->modes & modes) == 0x0) {
             EMSG(_(e_othermode));
             return NULL;
           }
+        }
+        if (*p == NUL) {  // found a full match
+          return menu;
+        }
         break;
       }
       menu = menu->next;
@@ -780,6 +785,7 @@ find_menu(vimmenu_T *menu, char_u * name, int modes)
       EMSG2(_(e_nomenu), name);
       return NULL;
     }
+    // Found a match, search the sub-menu.
     name = p;
     menu = menu->children;
   }
@@ -1235,7 +1241,7 @@ static char_u *popup_mode_name(char_u *name, int idx)
 ///
 /// @return a pointer to allocated memory.
 static char_u *menu_text(const char_u *str, int *mnemonic, char_u **actext)
-  FUNC_ATTR_NONNULL_RET FUNC_ATTR_MALLOC FUNC_ATTR_WARN_UNUSED_RESULT
+  FUNC_ATTR_NONNULL_RET FUNC_ATTR_WARN_UNUSED_RESULT
   FUNC_ATTR_NONNULL_ARG(1)
 {
   char_u      *p;

test/functional/ex_cmds/menu_spec.lua

@@ -362,6 +362,9 @@ describe('menu_get', function()
   it('matching path and modes', function()
     local m = funcs.menu_get("Test","i")
     local expected = {
+      {
+        shortcut = "T",
+        submenus = {
           {
             mappings = {
               i = {
@@ -370,7 +373,14 @@ describe('menu_get', function()
                 enabled = 1,
                 rhs = "insert",
                 silent = 0
-          }
+              },
+            },
+            priority = 500,
+            name = "Test",
+            hidden = 0
+          },
+          {
+          },
         },
         priority = 500,
         name = "Test",