BUILD: epoll/threads: Add test on MAX_THREADS to avoid warnings when complied without...

BUILD: epoll/threads: Add test on MAX_THREADS to avoid warnings when complied without threads

When HAProxy is complied without threads, gcc throws following warnings:

  src/ev_epoll.c:222:3: warning: array subscript is outside array bounds [-Warray-bounds]
  ...
  src/ev_epoll.c:199:11: warning: array subscript is outside array bounds [-Warray-bounds]
  ...

Of course, this is not a bug. In such case, tid is always equal to 0. But to
avoid the noise, a check on MAX_THREADS in "if (tid)" lines makes gcc happy.

This patch should be backported in 1.8 with the commit d9e7e36c ("BUG/MEDIUM:
epoll/threads: use one epoll_fd per thread").