impl: fix signed shrinker

Previously, it was possible for the signed shrinker to call abs on the
minimum value of an integer. This is incorrect and can lead to a panic,
so we avoid abs in that case.

We add a regression test that isn't guaranteed to trip the failure but
is highly likely to.

Fixes #268

src/arbitrary.rs

@@ -813,7 +813,7 @@ macro_rules! signed_shrinker {
                     } else {
                         let shrinker = SignedShrinker { x: x, i: x / 2 };
                         let mut items = vec![0];
-                        if shrinker.i < 0 {
+                        if shrinker.i < 0 && shrinker.x != <$ty>::MIN {
                             items.push(shrinker.x.abs());
                         }
                         Box::new(items.into_iter().chain(shrinker))

src/tester.rs

@@ -431,11 +431,21 @@ mod test {
         let expected_argument = format!("{:?}", [true, true]);
         assert_eq!(failing_case.arguments, vec![expected_argument]);
     }
+
     #[test]
     fn size_for_small_types_issue_143() {
         fn t(_: i8) -> bool {
             true
         }
         QuickCheck::new().gen(Gen::new(129)).quickcheck(t as fn(i8) -> bool);
     }
+
+    #[test]
+    fn regression_signed_shrinker_panic() {
+        fn foo_can_shrink(v: i8) -> bool {
+            let _ = crate::Arbitrary::shrink(&v);
+            true
+        }
+        crate::quickcheck(foo_can_shrink as fn(i8) -> bool);
+    }
 }