Fix soundness of FromBytes::read_from_io

See google#2319.

Author: kupiakos

src/lib.rs

@@ -4546,7 +4546,8 @@ pub unsafe trait FromBytes: FromZeros {
         Self: Sized,
         R: io::Read,
     {
-        let mut buf = CoreMaybeUninit::<Self>::zeroed();
+        let mut buf = CoreMaybeUninit::<Self>::uninit();
+        buf.zero();
         let ptr = Ptr::from_mut(&mut buf);
         // SAFETY: `buf` consists entirely of initialized, zeroed bytes.
         let ptr = unsafe { ptr.assume_validity::<invariant::Initialized>() };