Fix bug in GPU list balancing

The function split_sci_entry could produce empty lists, which can
cause illegal memory access or incorrect energies. Before commit
6106367b this bug was never triggered, since nsp_max was never smaller
than a full cj4 entry. But 6106367b introduced a but that could
produce negative nsp_max.

Fixes #1767.

Change-Id: I2007cf6851f94f4f2ca62f609a0628725014dbe7