Check that generator is successfully created before using

[chrisroberts]

This is just a simple little fix. I ran into this when I had misspelled PackageName in the yaml file. When the key was missing, the generator was not created and resulted in a panic:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x58 pc=0x5ff014]

goroutine 1 [running]:
github.com/xlab/c-for-go/generator.(*Generator).SetMaxMemory(0x0, 0x7439f5, 0xa)
        /home/spox/Projects/xlab/c-for-go/generator/generator.go:509 +0xb4
main.NewProcess(0x7ffdb591a37e, 0x9, 0x7ffdb591a375, 0x8, 0xc0000b6930, 0xc000099140, 0x42aad800)
        /home/spox/Projects/xlab/c-for-go/process.go:99 +0x409
main.main()
        /home/spox/Projects/xlab/c-for-go/main.go:79 +0x1c5

This just moves the error check prior to the usage of the generator which results in the expected behavior:

[ERR] no package name provided

[xlab]

Oof, thanks! That was some weird merge side-effect, so that line got above the error check. Sorry :)