Range larger than image results cause faulure

[broarr]

If --first-sync is larger than the number of images available that causes a panic. Need to truncate the slice to min(results, first_sync).

cargo run -- -m https://raw.githubusercontent.com/brown-bnc/bnc-resource-registry/master/singularity-manifest.yml --parallelize --force --first-sync 10 /gpfs/scratch/broarr/test
thread 'main' panicked at 'index 10 out of range for slice of length 9', src/main.rs:287:10
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace