solver: check completion before probing square

next_pos() returns the grid end sentinel when a placement completes the board. Avoid passing that sentinel to largest_square(), which requires an in-range position and otherwise reads past the grid.

Tests: Debug, Release, ASan, and UBSan CTest suites

Refs: #14

main.cc

@@ -297,10 +297,11 @@ namespace {
       sqs.push_back(sq);
 
       pos = grid.next_pos(pos + idx);
-      idx = grid.largest_square(pos, n);
 
       // Have we reached the end?  If so success!
       if (pos == grid.end()) { break; }
+
+      idx = grid.largest_square(pos, n);
     }
 
     return {length, sqs};