Fix iallocator crash when no solutions exist

Commit 54365762 added an un-guarded `head' call, which crashes with
“Prelude.head: empty list” when no results exists for the per-instance
allocation/relocation calls.

This patch fixes this, and also adds another check for an unguarded
`head' call during parsing of input data.
(cherry picked from commit e41f4ba0)