BUG/MEDIUM: lua: Fix segmentation fault if a Lua task exits

BUG/MEDIUM: lua: Fix segmentation fault if a Lua task exits

PiBa-NL reported that haproxy crashes with a segmentation fault
if a function registered using `core.register_task` returns.

An example Lua script that reproduces the bug is:

  mytask = function()
  	core.Info("Stopping task")
  end
  core.register_task(mytask)

The Valgrind output is as follows:

  ==6759== Process terminating with default action of signal 11 (SIGSEGV)
  ==6759==  Access not within mapped region at address 0x20
  ==6759==    at 0x5B60AA9: lua_sethook (in /usr/lib/x86_64-linux-gnu/liblua5.3.so.0.0.0)
  ==6759==    by 0x430264: hlua_ctx_resume (hlua.c:1009)
  ==6759==    by 0x43BB68: hlua_process_task (hlua.c:5525)
  ==6759==    by 0x4FED0A: process_runnable_tasks (task.c:231)
  ==6759==    by 0x4B2256: run_poll_loop (haproxy.c:2397)
  ==6759==    by 0x4B2256: run_thread_poll_loop (haproxy.c:2459)
  ==6759==    by 0x41A7E4: main (haproxy.c:3049)

Add the missing `task = NULL` for the `HLUA_E_OK` case. The error cases
have been fixed as of 253e53e661c49fb9723535319cf511152bf09bc7 which
first was included in haproxy v1.8-dev3. This bugfix should be backported
to haproxy 1.8.