Fix empty command panic, help for wrong "show" subcommand

[thaarok]

Current version will crash when I press Enter on empty command line.

genesis» 
panic: runtime error: index out of range [0] with length 0

goroutine 1 [running]:
main.main()
	/home/jkalina/Fantom/leveldb/leveldb-cli/main.go:84 +0xedc

Also when I try to use "show prefix" as "show (thePrefixIwantToFind)", no message is printed.

genesis» show myprefix
genesis»

This PR fixes both.

[liderman]

Hi, @hkalina!
Thanks for the fix