Fix `*Config.validate` method not to make a directory when the `-o` flag's path does not have a directory.

Fix `*Config.validate` method not to make a directory when the `-o` flag's path does not have a directory.

This PR fixes the following bug:

[Executed command]
go-bindata -o myfile.go data/

[Expected]
`myfile.go` will be created.

[Actual]
`myfile.go` was not created and the following message was shown:
bindata: Create output directory: mkdir : no such file or directory

[Reason]
Because `*Config.validate` method tried to make a directory even if the `-o` flag's path does not have a directory.