qmemman: fix early crash
clear_outdated_error_markers crashes if memory stats are not retrieved
yet. In practice it crashes at the very first call during daemon
startup, making the whole qmemman unusable.
This fixes bf4306b815a5c0e84ebcb8b58a3449fbac962f88
qmemman: clear "not responding" flags when VM require more memory
QubesOS/qubes-issues#3265