Fix DD load balancing bug with GPU sharing

The recent DD load balancing fix which solved the issue of incorrect
imbalance measure with GPU sharing (ba8232e9) addressed GPUs with
incorrect indexing. This caused out of bounds indexing in the GPU ID
query function. The query function also had a bug in the error checking
which allowed the incorrect indexing.
Now also mdrun -nb cpu -gpu_id ... is allowed, which before would give
a fatal error.

This commit addresses both issues; fixes #1385

Change-Id: I2800f610b873da92afe78bbfd869258f378ba2d7