port ModifyStatByNature fakematch fix (from #1179)

src/pokemon.c

@@ -5746,25 +5746,29 @@ u8 GetTrainerEncounterMusicId(u16 trainerOpponentId)
 
 u16 ModifyStatByNature(u8 nature, u16 n, u8 statIndex)
 {
-    // Dont modify HP, Accuracy, or Evasion by nature
+    u16 retVal;
+    // Don't modify HP, Accuracy, or Evasion by nature
     if (statIndex <= STAT_HP || statIndex > NUM_NATURE_STATS)
     {
-        // Should just be "return n", but it wouldn't match without this.
-        u16 retVal = n;
-        retVal++;
-        retVal--;
-        return retVal;
+        return n;
     }
 
     switch (gNatureStatTable[nature][statIndex - 1])
     {
     case 1:
-        return (u16)(n * 110) / 100; // NOTE: will overflow for n > 595 because the intermediate value is cast to u16 before the division. Fix by removing (u16) cast
+        retVal = n * 110;
+        retVal /= 100;
+        break;
     case -1:
-        return (u16)(n * 90) / 100;  // NOTE: will overflow for n > 728, see above
+        retVal = n * 90;
+        retVal /= 100;
+        break;
+    default:
+        retVal = n;
+        break;
     }
 
-    return n;
+    return retVal;
 }
 
 #define IS_LEAGUE_BATTLE                                                                \