Unequal: fix sense error in latin_solver_alloc fix.

In commit 5030d87903191d5 I gave latin_solver_alloc a return value,
and introduced a check of that value at every call site. One of the
checks was backwards, with the effect that Unequal game generation now
more or less always fails an assertion. For example:

$ unequal --generate 1 4#12345
unequal: unequal.c:1072: gg_best_clue: Assertion `best != -1' failed.

1 files changed, 1 insertions, 1 deletions

diff --git a/unequal.c b/unequal.c
index 0a2bafd..52041bb 100644
--- a/unequal.c
+++ b/unequal.c
@@ -890,7 +890,7 @@ static int solver_state(game_state *state, int maxdiff)
     struct latin_solver solver;
     int diff;
 
-    if (!latin_solver_alloc(&solver, state->nums, state->order))
+    if (latin_solver_alloc(&solver, state->nums, state->order))
         diff = latin_solver_main(&solver, maxdiff,
                                  DIFF_LATIN, DIFF_SET, DIFF_EXTREME,
                                  DIFF_EXTREME, DIFF_RECURSIVE,