configure loops prompt on null input

Configure asks for a licence in a loop. If a read error occurs, the loop is endless. If output is redirected to a file, the repeated prompts will ultimately fill the device.

To see how vicious it is, do the following:

1. start vi.
2. enter text: exec sh configure
3. enter command: !!sh

and wait until /tmp gets filled, the shell taking 95% CPU.