Minor optimization to not check if JSON number starts with '-' twice
Minor optimization to not check if JSON number starts with '-' twice

jmesmon/serde,erickt/serde,eefriedman/serde,thorbenk/serde,skade/serde,JohnHeitmann/serde,Hoverbear/serde,tempbottle/serde,csmulhern/serde,oli-obk/rust-serde,serde-rs/serde,3Hren/serde,Ticki/serde,sidred10/serde,infinityb/serde