Update Shared.cs (#234)

* Update Shared.cs

I think, I found the cause for the issue #139
This approach uses the byte buffer for looking for the signature, instead of reading it once with 64k, and then with 4byte steps again, if one byte matches the signature

* Update Shared.cs

* Update Shared.cs

make sure, splitted signatures, which won't fit as a whole in the buffer will be read again

src/Zip.Shared/Shared.cs

@@ -350,31 +350,26 @@ internal static long FindSignature(System.IO.Stream stream, int SignatureToFind)
             do
             {
                 n = stream.Read(batch, 0, batch.Length);
-                if (n != 0)
+                if (n >= 4)
                 {
-                    for (int i = 0; i < n; i++)
+                    for (int i = 0; i < n - 3; i++)
                     {
-                        if (batch[i] == targetBytes[3])
+                        if (batch[i] == targetBytes[3]
+                            && batch[i + 1] == targetBytes[2]
+                            && batch[i + 2] == targetBytes[1]
+                            && batch[i + 3] == targetBytes[0])
                         {
-                            long curPosition = stream.Position;
-                            stream.Seek(i - n, System.IO.SeekOrigin.Current);
-
-                            // workitem 7711
-                            int sig = ReadSignature(stream);
-
-                            success = (sig == SignatureToFind);
-                            if (!success)
-                            {
-                                stream.Seek(curPosition, System.IO.SeekOrigin.Begin);
-                            }
-                            else
-                                break; // out of for loop
+                            stream.Seek(i - n + 4, System.IO.SeekOrigin.Current);
+                            success = true;
+                            break; // out of for loop
                         }
                     }
                 }
                 else break;
                 if (success) break;
 
+                //Move back 3 bytes, to make sure incomplete signatures will be read as a whole
+                stream.Seek(-3, System.IO.SeekOrigin.Current);
             } while (true);
 
             if (!success)