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

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

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.

(cherry picked from commit 067f7dd17dc39144c4b292a13ceea53610b4b5e0)