Fix DSKIdentifier ModChecksum function

This never worked. Will probably fix a whole bunch of core-selection issues (including ones listed here: https://tasvideos.org/HomePages/CloakTheLurker/ZXHawk)

src/BizHawk.Emulation.Common/DSKIdentifier.cs

@@ -17,7 +17,7 @@ public class DskIdentifier
 		private string _possibleIdent = "";
 
 		/// <summary>
-		/// Default fallthrough to AppleII
+		/// Default fallthrough to AppleII - the AppleII *.dsk format seems to be very simple with no ident strings
 		/// </summary>
 		public string IdentifiedSystem { get; set; } = VSystemID.Raw.AppleII;
 
@@ -93,7 +93,7 @@ private void CalculateFormat()
 			// check for bootable status
 			if (trk.Sectors[0].SectorData != null && trk.Sectors[0].SectorData.Length > 0)
 			{
-				switch (trk.Sectors[0].GetChecksum256())
+				switch (trk.Sectors[0].GetModChecksum256())
 				{
 					case 3:
 						IdentifiedSystem = VSystemID.Raw.ZXSpectrum;
@@ -116,13 +116,13 @@ private void CalculateFormat()
 			}
 
 			// at this point the disk is not standard bootable
-			// try format analysis
+			// try format analysis			
 			if (trk.Sectors.Length == 9 && trk.Sectors[0].SectorSize == 2)
 			{
 				switch (trk.GetLowestSectorID())
 				{
 					case 1:
-						switch (trk.Sectors[0].GetChecksum256())
+						switch (trk.Sectors[0].GetModChecksum256())
 						{
 							case 3:
 								IdentifiedSystem = VSystemID.Raw.ZXSpectrum;
@@ -406,7 +406,15 @@ public class Sector
 			public byte[] SectorData { get; set; }
 			public bool ContainsMultipleWeakSectors { get; set; }
 
-			public int GetChecksum256() => SectorData.Sum(b => b % 256);
+			public int GetModChecksum256()
+			{
+				int res = 0;
+				for (int i = 0; i < ActualDataByteLength; i++)
+				{
+					res = (res + SectorData[i]) % 256;
+				}
+				return res;
+			}
 		}
 	}
 }