listener: unlock lev on error in listener_read_cb() Without this patch: $ regress --no-fork +listener/error_unlock listener/error_unlock: [warn] Error from accept() call: Too many open files [err] ../evthread.c:220: Assertion lock->count == 0 failed in ../evthread.c Aborted (core dumped) Fixes: #341 Fixes: listener/error_unlock