Add test to exercise InvalidUndefBytes

tests/compile-fail/undefined_buffer.rs

@@ -0,0 +1,20 @@
+// error-pattern: reading uninitialized memory
+
+use std::alloc::{alloc, dealloc, Layout};
+use std::slice::from_raw_parts;
+
+fn main() {
+    let layout = Layout::from_size_align(32, 8).unwrap();
+    unsafe {
+        let ptr = alloc(layout);
+        *ptr = 0x41;
+        *ptr.add(1) = 0x42;
+        *ptr.add(2) = 0x43;
+        *ptr.add(3) = 0x44;
+        *ptr.add(16) = 0x00;
+        let slice1 = from_raw_parts(ptr, 16);
+        let slice2 = from_raw_parts(ptr.add(16), 16);
+        drop(slice1.cmp(slice2));
+        dealloc(ptr, layout);
+    }
+}