nanovna_capture.py: NameError: name 'bytestream' is not defined

[Ho-Ro]

@krupis reported in NanoVNA-Saver/nanovna-saver#676:

C:\Users\petrikas.lu\Downloads\nanovna-tools-main\nanovna-tools-main>python nanovna_capture.py
Traceback (most recent call last):
  File "C:\Users\petrikas.lu\Downloads\nanovna-tools-main\nanovna-tools-main\nanovna_capture.py", line 115, in <module>
    if bytestream == b'capture?\r\nch> ': # error message
       ^^^^^^^^^^
NameError: name 'bytestream' is not defined

FIX: change bytestream to captured_bytes in this line

if len( captured_bytes ) != 2 * size:
    if bytestream == b'capture?\r\nch> ': # error message
        print( 'capture error - does the device support the "capture" cmd?' )
    else:
        print( 'capture error - wrong screen size?' )
    sys.exit()