rust: Fix null deref when casting (PR 23124)

Fix a null dereference when casting a value to a unit type.

ChangeLog
2018-04-28  Dan Robertson  <danlrobertson89@gmail.com>

	PR rust/23124
	* gdb/rust-exp.y (convert_params_to_types): Ensure that the params
	pointer is not null before dereferencing it.

testsuite/ChangeLog
2018-04-28  Dan Robertson  <danlrobertson89@gmail.com>

	PR rust/23124
	* gdb.rust/expr.exp: Test that the unit type is correctly parsed
	when casting.