Fix packet padding concatenation error (#1819)

[alexisbalbachan]

• This happens when trying to parse packets headers from raw data, if the available data is less than the expected header length the library will try to pad zeroes to it:

  impacket/impacket/ImpactPacket.py

  Lines 432 to 435 in 3ce41be

  	if(len(aBuffer) < hdr_len): #we must do something like this
  	diff = hdr_len - len(aBuffer)
  	for i in range(0, diff):
  	aBuffer += '\x00'

• The padding worked fine in python 2, the buffer and '\x00' are considered bytes.

• In python 3 '\x00' is considered a string so we get an error when trying to append it to a bytes buffer: TypeError: can't concat str to bytes

Affected classes so far are:

1. IP6
2. ICMP6
3. LINUXSLL
4. UDP
5. ARP
6. ICMP
7. IGMP

[alexisbalbachan]

Fixes #1819

[anadrianmanrique]

Thanks for the PR. Merging..