Fix trade exploit

Using the Max Sale button it was possible to "invert" the buying/selling state.

apps/openmw/mwgui/tradewindow.cpp

@@ -302,6 +302,9 @@ namespace MWGui
             }
         }
 
+        // Is the player buying?
+        bool buying = (mCurrentMerchantOffer < 0);
+
         if(mCurrentBalance > mCurrentMerchantOffer)
         {
             //if npc is a creature: reject (no haggle)
@@ -315,7 +318,7 @@ namespace MWGui
             int a = abs(mCurrentMerchantOffer);
             int b = abs(mCurrentBalance);
             int d = 0;
-            if (mCurrentBalance<0)
+            if (buying)
                 d = int(100 * (a - b) / a);
             else
                 d = int(100 * (b - a) / a);
@@ -336,7 +339,7 @@ namespace MWGui
             float pcTerm = (clampedDisposition - 50 + a1 + b1 + c1) * playerStats.getFatigueTerm();
             float npcTerm = (d1 + e1 + f1) * sellerStats.getFatigueTerm();
             float x = gmst.find("fBargainOfferMulti")->getFloat() * d + gmst.find("fBargainOfferBase")->getFloat();
-            if (mCurrentBalance<0)
+            if (buying)
                 x += abs(int(pcTerm - npcTerm));
             else
                 x += abs(int(npcTerm - pcTerm));