[binutils-gdb] Fix crash in quirk_rust_enum

[sergiodj+buildbot@sergiodj.net]

*** TEST RESULTS FOR COMMIT a037790ec570ae9f9bf535cbce25f238f90e8b4a ***

Author: Tom Tromey <tom@tromey.com>
Branch: master
Commit: a037790ec570ae9f9bf535cbce25f238f90e8b4a

Fix crash in quirk_rust_enum

I noticed that quirk_rust_enum can crash when presented with a union
whose fields are all scalar types.

This patch adds a new test case and fixes the bug.

Regression tested on Fedora 26 x86-64.

2018-04-17  Tom Tromey  <tom@tromey.com>

	* dwarf2read.c (quirk_rust_enum): Handle unions correctly.

2018-04-17  Tom Tromey  <tom@tromey.com>

	* gdb.rust/simple.rs (Union): New type.
	(main): New local "u".
	* gdb.rust/simple.exp (test_one_slice): Add new test case.