fix: get_param_ptr SIGSEGV

get_param_ptr(cmd->server->conf, C_USER, FALSE) returned NULL and cause
SIGSEGV on v1.3.4.

  Program received signal SIGSEGV, Segmentation fault.
  [Switching to Thread 0x7ffff7fe77c0 (LWP 4603)]
  0x00007ffff6e37451 in __strlen_sse2 () from /lib64/libc.so.6
  Missing separate debuginfos, use: debuginfo-install
  proftpd-1.3.4a-1.el6.rf.x86_64
  (gdb) bt
  #0  0x00007ffff6e37451 in __strlen_sse2 () from /lib64/libc.so.6
  #1  0x00007ffff5cf1653 in is_cache_exits (mmc=0x90f7d0, key=0x0) at
  mod_libmemcached_deny_blacklist.c:290
  #2  0x00007ffff5cf1996 in lmd_deny_blacklist_post_pass (cmd=0x91b368) at
  mod_libmemcached_deny_blacklist.c:384
  #3  0x0000000000434e22 in pr_module_call ()
  #4  0x000000000041558b in _dispatch ()
  #5  0x0000000000415e23 in pr_cmd_dispatch_phase ()
  #6  0x000000000041707c in cmd_loop ()
  #7  0x00000000004132c0 in fork_server ()
  #8  0x000000000041458a in daemon_loop ()
  #9  0x0000000000416676 in main ()