streaming: Fix map access in stream_manager::get_progress
streaming: Fix map access in stream_manager::get_progress

When the progress is queried, e.g., query from nodetool netstats
the progress info might not be updated yet.

Fix it by checking before access the map to avoid errors like:

std::out_of_range (_Map_base::at)

Fixes: #5437
Tests: nodetool_additional_test.py:TestNodetool.netstats_test

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