STLink: read16 exception in gdb_test

"integer out of range for 'I' format code" exception reported while running gdb_test.py. This was found in the test log attached to #480. Seems to be specific to 16-bit reads?

Traceback (most recent call last):
  File "C:\Users\Kwinten\Documents\New folder\pyOCD\pyocd\gdbserver\gdbserver.py", line 537, in handle_message
    reply = handler(msg[msgStart:])
  File "C:\Users\Kwinten\Documents\New folder\pyOCD\pyocd\gdbserver\gdbserver.py", line 886, in get_memory
    mem = self.target_context.read_memory_block8(addr, length)
  File "C:\Users\Kwinten\Documents\New folder\pyOCD\pyocd\debug\cache.py", line 512, in read_memory_block8
    return self._memcache.read_memory_block8(addr, size)
  File "C:\Users\Kwinten\Documents\New folder\pyOCD\pyocd\debug\cache.py", line 433, in read_memory_block8
    return self._context.read_memory_block8(addr, size)
  File "C:\Users\Kwinten\Documents\New folder\pyOCD\pyocd\debug\context.py", line 53, in read_memory_block8
    return self._core.read_memory_block8(addr, size)
  File "C:\Users\Kwinten\Documents\New folder\pyOCD\pyocd\coresight\cortex_m.py", line 566, in read_memory_block8
    data = self.ap.read_memory_block8(addr, size)
  File "C:\Users\Kwinten\Documents\New folder\pyOCD\pyocd\core\memory_interface.py", line 94, in read_memory_block8
    mem = self.read16(addr)
  File "C:\Users\Kwinten\Documents\New folder\pyOCD\pyocd\core\memory_interface.py", line 60, in read16
    return self.read_memory(addr, 16, now)
  File "C:\Users\Kwinten\Documents\New folder\pyOCD\pyocd\probe\stlink_probe.py", line 251, in read_memory
    result = conversion.byte_list_to_u16le_list(self._link.read_mem16(addr, 2, self._apsel))[0]
  File "C:\Users\Kwinten\Documents\New folder\pyOCD\pyocd\probe\stlink\stlink.py", line 284, in read_mem16
    return self._read_mem(addr, size, Commands.JTAG_READMEM_16BIT, self.MAXIMUM_TRANSFER_SIZE, apsel)
  File "C:\Users\Kwinten\Documents\New folder\pyOCD\pyocd\probe\stlink\stlink.py", line 222, in _read_mem
    cmd.extend(six.iterbytes(struct.pack('<IHB', addr, thisTransferSize, apsel)))
error: integer out of range for 'I' format code

Start in pyocd/probe/stlink/stlink.py at _read_mem, especially the struct.pack call shown in the traceback, and trace the 16-bit read path from pyocd/core/memory_interface.py. Run gdb_test.py with the STLink setup that reproduces the report. Done means 16-bit reads no longer raise the integer-range exception.