assert rather than panic

Co-authored-by: Oli Scherer <github35764891676564198441@oli-obk.de>

Author: RalfJung

compiler/rustc_const_eval/src/const_eval/eval_queries.rs

@@ -169,9 +169,7 @@ pub(super) fn op_to_const<'tcx>(
                 let (alloc_id, offset) = a.to_pointer(ecx).expect(msg).into_parts();
                 let alloc_id = alloc_id.expect(msg);
                 let data = ecx.tcx.global_alloc(alloc_id).unwrap_memory();
-                if offset.bytes() != 0 {
-                    panic!("{}", msg);
-                }
+                assert!(offset == Size::ZERO, "{}", msg);
                 let meta = b.to_target_usize(ecx).expect(msg);
                 ConstValue::Slice { data, meta }
             }