make sure that we don't read past the end of the compressed buffer

Bug: 10757
Change-Id: I30054c4a75ec86ea603cf78b702be5255c35f549
Reviewed-on: https://code.wireshark.org/review/5642
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>
Tested-by: Martin Kaiser <wireshark@kaiser.cx>