fix case where file has no extension and caused panic: runtime error: slice bounds out of range
fix case where file has no extension and caused panic: runtime error: slice bounds out of range

Signed-off-by: Joshua Rubin <5af9dd6785dff068189d7fd731f38e1d0699263c@zvelo.com>

progrium/gh-release,jimmidyson/gh-release,jimmidyson/gh-release,progrium/gh-release