patch 9.1.0682: Vim9: Segfault with uninitialized funcref

Problem:  Vim9: Segfault with uninitialized funcref
          (Daniel Viberg)
Solution: Check the Funcref for being Null before trying to access it
          (Ernie Rael)

fixes: #15523

Signed-off-by: Ernie Rael <errael@raelity.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/testdir/test_vim9_func.vim

@@ -164,6 +164,16 @@ def Test_wrong_function_name()
   END
   v9.CheckScriptFailure(lines, 'E1182:')
   delfunc g:Define
+
+  lines =<< trim END
+    vim9script
+    var F1_ref: func
+    def Start()
+      F1_ref()
+    enddef
+    Start()
+  END
+  v9.CheckScriptFailure(lines, 'E117:')
 enddef
 
 " Check that in a legacy script a :def accesses the correct script variables.

src/userfunc.c

@@ -2245,7 +2245,8 @@ find_func_imported(char_u *name, int flags)
 	    if (!HASHITEM_EMPTY(hi))
 	    {
 		dictitem_T *di = HI2DI(hi);
-		if (di->di_tv.v_type == VAR_FUNC)
+		if (di->di_tv.v_type == VAR_FUNC
+			&& di->di_tv.vval.v_string != NULL)
 		    func = find_func_even_dead(di->di_tv.vval.v_string, flags);
 	    }
 	}

src/version.c

@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    682,
 /**/
     681,
 /**/