Don't run doctests with invalid pointers

The `Handle::from_ptr` and `SystemTable<View>::from_ptr` doctests use a `0xdeadbeef` pointer, which of course is not valid. Change the tests to compile but not actually run so that they don't trigger UB. Noticed this when running the unit tests with Miri, which caught the undefined behavior the in the SystemTable case.

src/data_types/mod.rs

@@ -16,7 +16,7 @@ impl Handle {
  /// come from the Multiboot2 information structure or something similar.
  ///
  /// # Example
- /// ```rust
+ /// ```no_run
  /// use core::ffi::c_void;
  /// use uefi::Handle;
  ///

src/table/system.rs

@@ -78,7 +78,7 @@ impl<View: SystemTableView> SystemTable<View> {
  /// come from the Multiboot2 information structure or something similar.
  ///
  /// # Example
- /// ```rust
+ /// ```no_run
  /// use core::ffi::c_void;
  /// use uefi::prelude::{Boot, SystemTable};
  ///