add missing argument check in ulockmgr.c

add missing argument check in ulockmgr.c

Add missing argument check in ulockmgr.c to prevent calling
ulockmgr_server with illegal arguments. This would cause an ever growing
list of ulockmgr_server processes with an endless list of open files which
finally exceeds the open file handle limit.

It appears samba is sometimes calling flock with illegal / weired values.