scylla-gdb.py: get_text_range(): make compatible with >=3.0
scylla-gdb.py: get_text_range(): make compatible with >=3.0

The current method of obtaining the text range based on a known vptr
(`reactor::_backend`) was based on branch-2019.1, where
`reactor::_backend` is a value member. However in >=3.0
`reactor::_backend` is a `std::unique_ptr<>`. Adapt the code to work for
both.

Signed-off-by: Botond Dénes <a584fa4d75a0a85d721f44fb6e06315a2853b0b6@scylladb.com>
Message-Id: <20200310135957.86261-1-a584fa4d75a0a85d721f44fb6e06315a2853b0b6@scylladb.com>

avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla