Fixes #6 - Handling panic when passing empty slice to ReadImageBlob()

Since I was already into the codebase for other patches, I decided to submit this fix as well.

I used the second approach that was suggested, where the error is handled within the Go function. My opinion was that there was no point in making a CGO call when we know its going to fail. The error string mirrors what the C function would have returned in `GetLastError()`.