catalog/lease: fix nil pointer dereference in purgeOldVersions

Previously, we added logic that to handle errRenewLease due to session
expiration with a debug assertion making sure the latest version of a
descriptor was not historical. Sadly, the assertion did not take into
account concurrent clean up of the version being retained. To address
this, this patch adds an explicit nil check to handle this scenario.

Fixes: #154155

Release note: None

Author: fqazi

pkg/sql/catalog/lease/lease.go

@@ -1277,6 +1277,11 @@ func (m *Manager) purgeOldVersions(
 		}
 		// We encountered an error telling us to renew the lease.
 		newest := m.findNewest(id)
+		// It is possible that a concurrent drop / removal of this descriptor is
+		// occurring. If the newest version just doesn't exist, bail out.
+		if newest == nil {
+			break
+		}
 		// Assert this should never happen due to a fixed expiration, since the range
 		// feed is responsible for purging old versions and acquiring new versions.
 		if newest.hasFixedExpiration() {