fix(pcapng): an SHB or IDB shorter than its fixed fields rebuilds at full size #1422

Describe the bug
A Section Header Block or Interface Description Block shorter than its fixed fields rebuilds at the full fixed size. The parser over-reads past the declared Block Total Length.

To Reproduce
On main (ec2ba13a4), run PCAPNG.from_data(PCAPNG(raw).info).data:

• SHB with a body of only the byte-order magic (length 16): 16 → 28 octets.
• SHB with magic + version (length 20): 16 → 28 octets.
• IDB of length 12 (empty body) or 16 (linktype + reserved only): rebuilds as 20.
• A full SHB (28) and IDB (20) are exact.

Expected behavior
Byte-exact, per the every-round-trip ruling. Keep the block body as captured, as #1420 does for EPB/PB under 32 octets (block_raw).

Found by the #1420 cross-review. Blocked until #1420 merges, since both edit protocols/misc/pcapng.py.