ensure unpacking of proper length during decoding (#2664)

[brherger]

I am not sure if this fixes the issue with the garbage cleanup, but it seems like a correct and consistent thing to include to avoid the exception.

fixes #2664

[janiversen]

I am really confused, how can this help with the exception ???

The buffer needs to be >= 4, so the exception arises when the buffer is less than 4 bytes.

[brherger]

I have no way of knowing what is coming in, not logged in the exception. It will throw an exception for != 4.
This is also done elsewhere in these files.

>>> data =  b"\x11\x22\x33\x44\x55\x66"
>>> struct.unpack(">HH", data)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
struct.error: unpack requires a buffer of 4 bytes
>>> struct.unpack(">HH", data[:3])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
struct.error: unpack requires a buffer of 4 bytes
>>> struct.unpack(">HH", data[:4])
(4386, 13124)

[janiversen]

Seems it actually solves the case.

Thanks.

Ps. added test case.