Rollup merge of #99198 - RalfJung:alloc-null-ptr, r=JohnTitor

add missing null ptr check in alloc example

`alloc` can return null on OOM, if I understood correctly. So we should never just deref a pointer we get from `alloc`.

library/alloc/src/alloc.rs

@@ -70,11 +70,14 @@
 /// # Examples
 ///
 /// ```
-/// use std::alloc::{alloc, dealloc, Layout};
+/// use std::alloc::{alloc, dealloc, handle_alloc_error, Layout};
 ///
 /// unsafe {
 ///     let layout = Layout::new::<u16>();
 ///     let ptr = alloc(layout);
+///     if ptr.is_null() {
+///         handle_alloc_error(layout);
+///     }
 ///
 ///     *(ptr as *mut u16) = 42;
 ///     assert_eq!(*(ptr as *mut u16), 42);