Don't mix up capacity and length

- `make([]a, b)` create a slice of `b` elements `a`
- `make([]a, b, c)` create a slice of `0` elements `a`, but reserve space for `c` of them

When using `append` on the former, it will result on a slice with `b` leading
elements, which is unlikely to be what we want. This commit replaces the two
instances where this happens with the latter construct.

internal/storage/enclosure.go

@@ -132,7 +132,7 @@ func (s *Storage) updateEnclosures(tx *sql.Tx, entry *model.Entry) error {
 		return nil
 	}
 
-	sqlValues := make([]string, len(entry.Enclosures))
+	sqlValues := make([]string, 0, len(entry.Enclosures))
 	for _, enclosure := range entry.Enclosures {
 		sqlValues = append(sqlValues, strings.TrimSpace(enclosure.URL))