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