FTDISerialDevice: simplify adaptArray() length formula and fix crash bug

The old iterative method of calculating realLength was pretty clumsy
and slow.

Additionally, it failed to consider an incomplete FTDI header.  This
shouldn't happen, but it was seen in the dirty real world:

 java.lang.ArrayIndexOutOfBoundsException: src.length=65 srcPos=2 dst.length=61 dstPos=0 length=62
   at java.lang.System.arraycopy(Native Method)
   at com.felhr.usbserial.FTDISerialDevice.copyData(FTDISerialDevice.java:527)
   at com.felhr.usbserial.FTDISerialDevice.adaptArray(FTDISerialDevice.java:508)
   at com.felhr.usbserial.UsbSerialDevice$WorkerThread.doRun(UsbSerialDevice.java:359)
   at com.felhr.usbserial.AbstractWorkerThread.run(AbstractWorkerThread.java:21)

The same bug would manifest in copyData() due to a badly implemented
length check.

Fixes https://github.com/XCSoar/XCSoar/issues/1032

usbserial/src/main/java/com/felhr/usbserial/FTDISerialDevice.java

@@ -495,15 +495,11 @@ public class FTDISerialDevice extends UsbSerialDevice
         int length = ftdiData.length;
         if(length > 64)
         {
-            int n = 1;
-            int p = 64;
             // Precalculate length without FTDI headers
-            while(p < length)
-            {
-                n++;
-                p = n*64;
-            }
-            int realLength = length - n*2;
+            int realLength = (length / 64) * 62;
+            if (length % 64 > 2)
+                realLength += length % 64 - 2;
+
             byte[] data = new byte[realLength];
             copyData(ftdiData, data);
             return data;
@@ -529,7 +525,7 @@ public class FTDISerialDevice extends UsbSerialDevice
             dstPos += 62;
         }
         int remaining = src.length - srcPos + 2;
-        if (remaining > 0)
+        if (remaining > 2)
         {
             System.arraycopy(src, srcPos, dst, dstPos, remaining - 2);
         }