BUG/MINOR: illegal use of the malloc_trim() function if jemalloc is used

BUG/MINOR: illegal use of the malloc_trim() function if jemalloc is used

In the event that HAProxy is linked with the jemalloc library, it is still
shown that malloc_trim() is enabled when executing "haproxy -vv":
  ..
  Support for malloc_trim() is enabled.
  ..

It's not so much a problem as it is that malloc_trim() is called in the
pat_ref_purge_range() function without any checking.

This was solved by setting the using_default_allocator variable to the
correct value in the detect_allocator() function and before calling
malloc_trim() it is checked whether the function should be called.