updated for version 7.3.146

Problem:    It's possible to assign to a read-only member of a dict.
            It's possible to create a global variable "0". (ZyX)
            It's possible to add a v: variable with ":let v:.name = 1".
Solution:   Add check for dict item being read-only.
            Check the name of g: variables.
            Disallow adding v: variables.