Adjust fuel consumption to be empty when fuel is at 0 rather than 1

[itsrainy]

I ran into this when attempting to use set_fuel in a fuzz target (bytecodealliance/wasm-tools#769). set_fuel calls consume_fuel(0) in order to get the current amount of fuel in a Store. However, consume_fuel(0) will return an out of fuel error if there is 0 fuel remaining, which is the case with a new Store.

Switching the < to a <= also has the effect of making this work:

...
let mut store = Store::new(&engine, ());
store.add_fuel(1).unwrap();
store.consume_fuel(1).unwrap();

previously this would have returned an "out of fuel" error, which doesn't seem correct, though maybe there's some nuance I'm missing?

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[alexcrichton]

Seems reasonable to me, thanks!