Fix potential errors in SQLite code

Signed-off-by: Kévin Commaille <zecakeh@tedomum.fr>

Author: zecakeh

crates/matrix-sdk-sqlite/src/event_cache_store.rs

@@ -279,10 +279,20 @@ impl EventCacheStore for SqliteEventCacheStore {
 
             // Finally, if the cache size is too big, remove old items until it fits.
             if let Some(max_cache_size) = policy.max_cache_size {
-                let cache_size: usize =
-                    txn.query_row("SELECT sum(length(data)) FROM media", (), |row| row.get(0))?;
-
-                if cache_size > max_cache_size {
+                // i64 is the integer type used by SQLite, use it here to avoid usize overflow
+                // during the conversion of the result.
+                let cache_size_int = txn
+                    .query_row("SELECT sum(length(data)) FROM media", (), |row| {
+                        // `sum()` returns `NULL` if there are no rows.
+                        row.get::<_, Option<i64>>(0)
+                    })?
+                    .unwrap_or_default();
+                let cache_size_usize = usize::try_from(cache_size_int);
+
+                // If the cache size is overflowing or bigger than max cache size, clean up.
+                if cache_size_usize.is_err()
+                    || cache_size_usize.is_ok_and(|cache_size| cache_size > max_cache_size)
+                {
                     // Get the sizes of the media contents ordered by last access.
                     let mut cached_stmt = txn.prepare_cached(
                         "SELECT rowid, length(data) FROM media ORDER BY last_access DESC",