compiler: check for nil receiver in value method

compiler: check for nil receiver in value method

    We already dereference the pointer to copy the value, but if the
    method does not use the value then the pointer dereference may be
    optimized away.  Do an explicit nil check so that we get the panic
    that is required.

    Fixes golang/go#19806

    Reviewed-on: https://go-review.googlesource.com/91275

	* go.go-torture/execute/printnil.go: New test.

From-SVN: r257280