scylla-gdb/test_misc.py::test_active_sstables is flaky: gdb.MemoryError: Cannot access memory at address 0x300000000000000

[tgrabiec]

=================================== FAILURES ===================================
_____________________________ test_active_sstables _____________________________

gdb = <module 'gdb' from '/usr/share/gdb/python/gdb/__init__.py'>

    def test_active_sstables(gdb):
>       scylla(gdb, 'active-sstables')

test_misc.py:43: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

gdb = <module 'gdb' from '/usr/share/gdb/python/gdb/__init__.py'>
cmd = 'active-sstables'

    def scylla(gdb, cmd):
>       return gdb.execute('scylla ' + cmd, from_tty=False, to_string=True)
E       gdb.error: Error occurred in Python: Cannot access memory at address 0x300000000000000

test_misc.py:7: error
----------------------------- Captured stderr call -----------------------------
Traceback (most recent call last):
  File "/jenkins/workspace/scylla-master/scylla-ci/scylla/test/scylla-gdb/../../scylla-gdb.py", line 1884, in invoke
    for sst in find_active_sstables():
  File "/jenkins/workspace/scylla-master/scylla-ci/scylla/test/scylla-gdb/../../scylla-gdb.py", line 1835, in find_active_sstables
    yield sstable_ptr.reinterpret_cast(sstable_ptr_type)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
gdb.MemoryError: Cannot access memory at address 0x300000000000000

Possible cause is that find_single_sstable_readers() yields an object which is mistakenly identified as sstable reader, and hence its _sst._p field doesn't actually point to an sstable object but is some other data.