fix(ext/flash): Fix panic when JS caller doesn't consume request body (#16173)

If the JS handler gets a POST, PUT, or PATCH request, but doesn't
`await` the body, deno would panic because it will try to read the body
even though the request has already been handled.

Not sure how/where to test this case, so I could use some help with
that.

ext/flash/lib.rs

@@ -754,7 +754,11 @@ async fn op_flash_read_body(
     .as_mut()
     .unwrap()
   };
-  let tx = ctx.requests.get_mut(&token).unwrap();
+  let tx = match ctx.requests.get_mut(&token) {
+    Some(tx) => tx,
+    // request was already consumed by caller
+    None => return 0,
+  };
 
   if tx.te_chunked {
     let mut decoder =