update fn ptr tests

Author: RalfJung

tests/compile-fail/validity/fn_ptr_offset.rs

@@ -1,5 +1,5 @@
 #![allow(invalid_value)]
 
 fn main() {
-    let _b: fn() = unsafe { std::mem::transmute(0usize) }; //~ ERROR encountered a potentially null function pointer
+    let _b: fn() = unsafe { std::mem::transmute(0usize) }; //~ ERROR encountered a null function pointer
 }

tests/compile-fail/validity/invalid_fnptr_null.rs

@@ -62,9 +62,10 @@ fn main() {
     // Any non-null value is okay for function pointers.
     unsafe {
         let _x: fn() = mem::transmute(1usize);
-        let mut b = Box::new(42);
-        let ptr = &mut *b as *mut _;
+        let mut b = Box::new(42u8);
+        let ptr = &mut *b as *mut u8;
         drop(b);
         let _x: fn() = mem::transmute(ptr);
+        let _x: fn() = mem::transmute(ptr.wrapping_offset(1));
     }
 }