shearwater: fix userdata corruption in predator parser samples loop

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).

Signed-off-by: Michael Keller <github@ike.ch>

Author: mikeller

src/shearwater_predator_parser.c

@@ -1224,7 +1224,7 @@ shearwater_predator_parser_samples_foreach (dc_parser_t *abstract, dc_sample_cal
 				if ((status & PPO2_EXTERNAL) == 0) {
 					double calculated_ppo2 = data[offset + pnf + 6] / 100.0;
 
-					if (userdata) {
+					if (!callback && userdata) {
 						struct dc_parser_sensor_calibration_t *out = (struct dc_parser_sensor_calibration_t *)userdata;
 
 						out->external_ppo2_used = true;