patch 8.2.1764: Vim9: no error when assigning to script var with wrong type

Problem:    Vim9: no error when assigning to script var with wrong type.
Solution:   Fix off-by-one error. (closes #7028)
diff --git a/src/testdir/test_vim9_assign.vim b/src/testdir/test_vim9_assign.vim
index 5d6dec1..cd3ba8c 100644
--- a/src/testdir/test_vim9_assign.vim
+++ b/src/testdir/test_vim9_assign.vim
@@ -567,6 +567,16 @@
     assert_equal(44, t)
   END
   CheckScriptSuccess(lines)
+
+  lines =<< trim END
+      vim9script
+      var n: number
+      def Func()
+        n = 'string'
+      enddef
+      defcompile
+  END
+  CheckScriptFailure(lines, 'E1012: Type mismatch; expected number but got string')
 enddef
 
 def Mess(): string