0-offset is also not always okay

Author: RalfJung

tests/compile-fail/ptr_offset_ptr_plus_0.rs

@@ -0,0 +1,7 @@
+// error-pattern: outside bounds of allocation
+
+fn main() {
+    let x = Box::into_raw(Box::new(0u32));
+    let x = x.wrapping_offset(8); // okay, this has no inbounds tag
+    let _x = unsafe { x.offset(0) }; // UB despite offset 0, the pointer is not inbounds of the only object it can point to
+}