mds/Migrator.cc: fix dereference after null check

Add assert for 'le' to fix:

CID 716923 (#1 of 1): Dereference after null check (FORWARD_NULL)
  var_deref_model: Passing null pointer "le" to function
  "LogEvent::get_start_off() const", which dereferences it.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>