Fix a compiler warning.

[sunfishcode]

Fix the following warning from Rust 1.35:

warning: cannot borrow `*self` as mutable because it is also borrowed as immutable
   --> wasmtime-runtime/src/instance.rs:473:25
    |
465 |         } else if let Some(start_export) = self.module.exports.get("_start") {
    |                                            ----------- immutable borrow occurs here
...
473 |                         self.invoke_function(*func_index)
    |                         ^^^^                 ----------- immutable borrow later used here
    |                         |
    |                         mutable borrow occurs here
    |
    = note: #[warn(mutable_borrow_reservation_conflict)] on by default
    = warning: this borrowing pattern was not meant to be accepted, and may become a hard error in the future
    = note: for more information, see issue #59159 <https://github.com/rust-lang/rust/issues/59159>

[kubkon]

Nice one! I was meant to do that one myself, but didn't get the chance yet.