Fix usbmux to return any received data.

When receiving data from the other end in usbmux mode, we need to return any
data we get, not wait to fill the input buffer.

Otherwise any clients waiting for data will wait forever: in fact the initial
handshake would fail to complete, because we wouldn't pass along the 28-byte
success response because we were trying to fill the 1028-byte buffer with
data.

Also add some input validation to make sure we don't write data into random
memory + take the requested offset into account when writing data into the
output array.

src/Tools/dotnet-dsrouter/USBMuxTcpClientRouterFactory.cs

@@ -118,12 +118,18 @@ public override Task FlushAsync(CancellationToken cancellationToken)
 
         public override int Read(byte[] buffer, int offset, int count)
         {
-            bool continueRead = true;
-            int bytesToRead = count;
-            int totalBytesRead = 0;
-            int currentBytesRead = 0;
+            int bytesRead = 0;
 
-            while (continueRead && bytesToRead - totalBytesRead > 0)
+            if (offset + count > buffer.Length)
+                throw new InvalidOperationException ($"Potential write beyond end of buffer");
+
+            if (offset < 0)
+                throw new InvalidOperationException ($"Write before beginning of buffer");
+
+            if (count < 0)
+                throw new InvalidOperationException ($"Negative read count");
+
+            while (true)
             {
                 if (!IsOpen)
                     throw new EndOfStreamException();
@@ -132,21 +138,15 @@ public override int Read(byte[] buffer, int offset, int count)
                 {
                     fixed (byte* fixedBuffer = buffer)
                     {
-                        currentBytesRead = USBMuxInterop.recv(_handle, fixedBuffer + totalBytesRead, new IntPtr(bytesToRead - totalBytesRead), 0);
+                        bytesRead = USBMuxInterop.recv(_handle, fixedBuffer + offset, new IntPtr (count), 0);
                     }
                 }
 
-                if (currentBytesRead == -1 && Marshal.GetLastWin32Error() == USBMuxInterop.EINTR)
+                if (bytesRead == -1 && Marshal.GetLastWin32Error() == USBMuxInterop.EINTR)
                     continue;
 
-                continueRead = currentBytesRead > 0;
-                if (!continueRead)
-                    break;
-
-                totalBytesRead += currentBytesRead;
+                return bytesRead;
             }
-
-            return totalBytesRead;
         }
 
         public override Task<int> ReadAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken)