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.

1 files changed, 7 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.rust/simple.rs b/gdb/testsuite/gdb.rust/simple.rs
index b2b5dfe..e5bbe52 100644
--- a/gdb/testsuite/gdb.rust/simple.rs
+++ b/gdb/testsuite/gdb.rust/simple.rs
@@ -80,6 +80,11 @@ struct ParametrizedStruct<T> {
     value: T
 }
 
+union Union {
+    f1: i8,
+    f2: u8,
+}
+
 fn main () {
     let a = ();
     let b : [i32; 0] = [];
@@ -153,6 +158,8 @@ fn main () {
         value: 0,
     };
 
+    let u = Union { f2: 255 };
+
     println!("{}, {}", x.0, x.1);        // set breakpoint here
     println!("{}", diff2(92, 45));
     empty();