Panic when x86db-gogen is run without args

[quasilyte]

When len(os.Args) < 2, program panics:

$ ./bin/x86db-gogenpanic: runtime error: slice bounds out of range

goroutine 1 [running]:
main.main()
	~/x86db_bin/src/github.com/dlespiau/x86db/cmd/x86db-gogen/main.go:213 +0x984

Can be fixed with a simple check in the beginning of main.