add an undef validation test

tests/compile-fail/validation_undef.rs

@@ -0,0 +1,14 @@
+#![allow(unused_variables)]
+// error-pattern: attempted to read undefined bytes
+
+mod safe {
+    use std::mem;
+
+    pub(crate) fn make_float() -> f32 {
+        unsafe { mem::uninitialized() }
+    }
+}
+
+fn main() {
+    let _x = safe::make_float();
+}