core: avoid crash when reading corrupted git data

If a merge mishap creates inconsistent data for a dive in git storage,
where the dive references a dive site that no longer exists, the app
would crash when trying to open the cloud storage.

I don't think a NULL dive could ever happen, but this seems fairly cheap
insurance.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

CHANGELOG.md

@@ -1,3 +1,4 @@
+- core: avoid crash with corrupted cloud storage
 - planner: make ESC (cancel plan) work when moving handles
 - dive list: make dive guide visible in dive list [#3382]
 - general: rename dive master to dive guide

core/divesite.c

@@ -371,6 +371,14 @@ void purge_empty_dive_sites(struct dive_site_table *ds_table)
 void add_dive_to_dive_site(struct dive *d, struct dive_site *ds)
 {
 	int idx;
+	if (!d) {
+		fprintf(stderr, "Warning: add_dive_to_dive_site called with NULL dive\n");
+		return;
+	}
+	if (!ds) {
+		fprintf(stderr, "Warning: add_dive_to_dive_site called with NULL dive site\n");
+		return;
+	}
 	if (d->dive_site == ds)
 		return;
 	if (d->dive_site) {