Avoid panic when calling Elem on a nil map member.

[robshakir]

 * (M) ygot/struct_validation_map(_test)?.go
   - if a map was received that had a nil value in it, this caused
     a panic to occur since `(reflect.Value).Elem()` does not handle
     these cases cleanly. Add a check whether the value is nil before
     calling Elem.

[coveralls]

coverage: 89.612% (+0.002%) from 89.61%
when pulling b7a874c on copy-reflect-panic
into 4dcc65e on master.