Rollup merge of #142741 - a1phyr:fix_unsoundness, r=Mark-Simulacrum

Fix unsoundness in some tests
These tests were marked uninit bytes as initilized, which is unsound. Use initialized `MaybeUninit` instead.

Author: GuillaumeGomez

library/coretests/tests/io/borrowed_buf.rs

@@ -66,7 +66,7 @@ fn clear() {
 
 #[test]
 fnset_init(){
-let buf:&mut[_] = &mut[MaybeUninit::uninit();16];
+let buf:&mut[_] = &mut[MaybeUninit::zeroed();16];
 letmut rbuf:BorrowedBuf<'_> = buf.into();
 
 unsafe{
@@ -134,7 +134,7 @@ fn reborrow_written() {
 
 #[test]
 fncursor_set_init(){
-let buf:&mut[_] = &mut[MaybeUninit::uninit();16];
+let buf:&mut[_] = &mut[MaybeUninit::zeroed();16];
 letmut rbuf:BorrowedBuf<'_> = buf.into();
 
 unsafe{