frag6: Fix invalid 32-bit versus 64-bit printouts

[fxlb]

Print "[length < 0] (invalid)" when fragment length is negative.

In this case, e.g. when IPv6 payload length is zero, the 32-bit and 64-bit printouts were different.

Example:
32-bit:
frag (0x80000074:1280|4294967288)
---
64-bit:
frag (0x80000074:1280|18446744073709551608)

Add a test file.

[infrastation]

Two Buildbot workers experienced issues unrelated to the changes, this is now fixed.