Fix out of bound error if material name is missing

[Dyedbyte]

When reading material names specified by the usemtl tag, the program may panic due to an out of bound bug if the material name is missing. This PR should fix this error by splitting the line at the first whitespace so that everything after the tag (including whitespace) belongs to the material name.

Minimal reproduction

use tobj::{LoadOptions, load_obj_buf, load_mtl_buf};

fn main() {
    let mut data = "usemtl".as_bytes();
    load_obj_buf(&mut data, &LoadOptions::default(), |_| load_mtl_buf(&mut "".as_bytes()));
}

[Twinklebear]

Thanks @Dyedbyte , this looks good! It looks like the last thing to do before merging this is to run cargo fmt to fix the job: https://github.com/Twinklebear/tobj/runs/5370142090?check_suite_focus=true . The nightly failure will take some other fixes since it looks like some functionality has changed there so that's ok to be failing.

[Dyedbyte]

@Twinklebear I have now run cargo fmt, I hope the formatting looks now as expected.

[Twinklebear]

Looks great, thanks @Dyedbyte !