bolts: Fix UB in Truncate trait (#3207)

* bolts: Fix UB in Truncate trait

* fix test

* rename test

* fmt

libafl_bolts/src/ownedref.rs

@@ -97,9 +97,12 @@ impl<T> Truncate for &[T] {
 
 impl<T> Truncate for &mut [T] {
     fn truncate(&mut self, len: usize) {
-        let mut value = core::mem::take(self);
-        value = unsafe { value.get_unchecked_mut(..len) };
-        let _: &mut [T] = core::mem::replace(self, value);
+        let value = core::mem::take(self);
+        let len = value.len().min(len);
+        let truncated = value
+            .get_mut(..len)
+            .expect("Truncate with len <= len() should always work");
+        let _: &mut [T] = core::mem::replace(self, truncated);
     }
 }
 
@@ -1232,3 +1235,26 @@ where
         }
     }
 }
+
+#[cfg(test)]
+mod test {
+    use crate::Truncate;
+
+    #[test]
+    fn test_truncate() {
+        let mut data = [0; 1024];
+        let mut slice = &mut data as &mut [_];
+
+        slice.truncate(1);
+        assert_eq!(0, slice[0]);
+        assert_eq!(1, slice.len());
+
+        slice.truncate(100);
+        assert_eq!(0, slice[0]);
+        assert_eq!(1, slice.len());
+
+        slice.truncate(0);
+        slice.truncate(100);
+        assert_eq!(0, slice.len());
+    }
+}