fix miri reported UB with direct-threading

Author: Robbepop

crates/wasmi/src/engine/executor/handler/dispatch.rs

@@ -22,8 +22,12 @@ pub fn fetch_handler(ip: Ip) -> (Ip, Handler) {
             (ip, handler)
         }
         false => {
-            let (ip, handler) = unsafe { ip.decode::<usize>() };
-            let handler = unsafe { ::core::mem::transmute::<usize, Handler>(handler) };
+            let (ip, addr) = unsafe { ip.decode::<usize>() };
+            let handler = unsafe {
+                ::core::mem::transmute::<*const (), Handler>(
+                    ::core::ptr::with_exposed_provenance(addr)
+                )
+            };
             (ip, handler)
         }
     }