Fix duplicate levels in levelstats if you don't have a higher score

This is a regression from 25779606b4
(PR #74).

On the one hand, I should've thought this through carefully when
implementing the fix for the lower-score overwrite bug. On the other
hand, flibit didn't seem to notice either. And on the third hand, no one
else seems to have noticed, when they have had over 8 months to do so.
Not even the person who originally reported the lower-score overwrite
bug and also reported other bugs I hadn't noticed (e.g. the "You have
rescued a crewmate!" in Flip Mode) noticed this bug, which I believe was
weee50. But to be fair, he does seem to be less active nowadays.

On the fourth hand, I only realized the cause of the duplicate bug after
stepping through it in GDB, instead of just looking at it and going "hey
wait a minute" earlier. I'm surprised it didn't take me longer to
realize the problem.

I'm not sure what all these hands mean anymore, or where I'm getting
extra hands from. Whatever. This regression is fixed now.

desktop_version/src/Game.cpp

@@ -438,10 +438,14 @@ void Game::updatecustomlevelstats(std::string clevel, int cscore)
             break;
         }
     }
-    if(tvar>=0 && cscore > customlevelstats[tvar].score)
+    if(tvar>=0)
     {
-        //update existing entry
-        customlevelstats[tvar].score=cscore;
+        // We have an existing entry
+        // Don't update it unless it's a higher score
+        if (cscore > customlevelstats[tvar].score)
+        {
+            customlevelstats[tvar].score=cscore;
+        }
     }
     else
     {