http2: fix leak with range files

http2: fix leak with range files

Ticket: #5808

May have been introduced by a24d7dc45c818054f97448ce42ca9ba270b3b8e4

Function http2_range_open expects to be called only when
tx.file_range is nil. One condition to ensure this is to check
that we are beginning the files contents. The filetracker field
file_open is not fit for this, as it may be reset to false.