test that 0 cannot be offset-inbounds by 0

Author: RalfJung

tests/compile-fail/ptr_offset_0_plus_0.rs

@@ -0,0 +1,7 @@
+// error-pattern: outside bounds of allocation
+
+fn main() {
+    let x = 0 as *mut i32;
+    let _x = x.wrapping_offset(8); // ok, this has no inbounds tag
+    let _x = unsafe { x.offset(0) }; // UB despite offset 0, NULL is never inbounds
+}