Special case in dead-end checking which prevents the solver falling

Special case in dead-end checking which prevents the solver falling
over on a grid containing a 0 (completely blank) tile. This can't
happen in self-generated grids, but can happen if you type in a grid
from another Net implementation. Previously, the solver would notice
(technically correctly!) that a completely blank tile connects to no
other tiles and thus forms an isolated subgraph, and would therefore
complain that no orientation of that tile could possibly yield a
valid solution...

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@5828 cda61777-01e9-0310-a592-d414129be87e