shearwater: fix userdata corruption in predator parser samples loop

[mikeller]

shearwater_predator_parser_samples_foreach serves two purposes:
an internal calibration pre-pass called with callback=NULL and a
dc_parser_sensor_calibration_t * as userdata, and normal sample
enumeration called with a real callback and the caller's own userdata.

The guard that restricted calibration writes to the pre-pass checked
only that userdata was non-NULL, so during normal sample enumeration it
cast the caller's userdata pointer to dc_parser_sensor_calibration_t *
and wrote into it. For any CCR dive with internal (non-external) ppo2,
this wrote the value 1 into the first byte of whatever struct the caller
passed, e.g. the low byte of divecomputer::when in Subsurface (a small
timestamp corruption) or a FILE * member in dctool (a crash).

Fix: add !callback to the guard so the internal calibration writes only
happen during the pre-pass (callback == NULL).

Fixes #127.

[Copilot]

🟢 Approval recommended

The change correctly limits calibration writes to the documented pre-pass path (callback == NULL) and the calibration pre-pass call site is present and consistent with this contract.

Pull request overview

Fixes a bug in the Shearwater Predator samples loop where the function’s dual-use of userdata (calibration pre-pass vs. normal enumeration) could corrupt caller-provided userdata during normal sample callbacks.

Changes:

• Restrict internal calibration writes to only occur during the calibration pre-pass by additionally requiring callback == NULL.

File summaries

File	Description
src/shearwater_predator_parser.c	Gates calibration-struct writes on !callback to prevent writing into caller userdata during normal sample enumeration.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 0
• Review effort level: Lite

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