Test and initial fix for crash with dictwatcherdel

Fixes https://github.com/neovim/neovim/issues/11188.

src/nvim/eval/typval.c

@@ -1109,6 +1109,7 @@ void tv_dict_watcher_add(dict_T *const dict, const char *const key_pattern,
   watcher->key_pattern_len = key_pattern_len;
   watcher->callback = callback;
   watcher->busy = false;
+  watcher->needs_free = false;
   QUEUE_INSERT_TAIL(&dict->watchers, &watcher->node);
 }
 
@@ -1197,7 +1198,11 @@ bool tv_dict_watcher_remove(dict_T *const dict, const char *const key_pattern,
   }
 
   QUEUE_REMOVE(w);
-  tv_dict_watcher_free(watcher);
+  if (watcher->busy) {
+    watcher->needs_free = true;
+  } else {
+    tv_dict_watcher_free(watcher);
+  }
   return true;
 }
 
@@ -1268,6 +1273,9 @@ void tv_dict_watcher_notify(dict_T *const dict, const char *const key,
       callback_call(&watcher->callback, 3, argv, &rettv);
       watcher->busy = false;
       tv_clear(&rettv);
+      if (watcher->needs_free) {
+        tv_dict_watcher_free(watcher);
+      }
     }
   }
   tv_dict_unref(dict);

src/nvim/eval/typval.h

@@ -89,6 +89,7 @@ typedef struct dict_watcher {
   size_t key_pattern_len;
   QUEUE node;
   bool busy;  // prevent recursion if the dict is changed in the callback
+  bool needs_free;
 } DictWatcher;
 
 /// Bool variable values

test/functional/ex_cmds/dict_notifications_spec.lua

@@ -371,4 +371,24 @@ describe('VimL dictionary notifications', function()
     eq(1, eval('g:called'))
   end)
 
+  it('does not crash when using dictwatcherdel in callback', function()
+    source([[
+      let g:d = {}
+
+      function! W(...)
+        call dictwatcherdel(g:d, '*', function('W'))
+        try
+          call dictwatcherdel({}, 'meh', function('tr'))
+        catch
+          let g:exc = v:exception
+        endtry
+      endfunction
+
+      call dictwatcheradd(g:d, '*', function('W'))
+      let g:d.foo = 23
+    ]])
+    eq(23, eval('g:d.foo'))
+    eq("Vim(call):Couldn't find a watcher matching key and callback", eval('g:exc'))
+  end)
+
 end)