BUG/MINOR: hlua: Fix memory leak in hlua_alloc

BUG/MINOR: hlua: Fix memory leak in hlua_alloc

During a configuration check valgrind reports:

    ==14425== 0 bytes in 106 blocks are definitely lost in loss record 1 of 107
    ==14425==    at 0x4C2DB8F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
    ==14425==    by 0x4C2FDEF: realloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
    ==14425==    by 0x443CFC: hlua_alloc (hlua.c:8662)
    ==14425==    by 0x5F72B11: luaM_realloc_ (in /usr/lib/x86_64-linux-gnu/liblua5.3.so.0.0.0)
    ==14425==    by 0x5F78089: luaH_free (in /usr/lib/x86_64-linux-gnu/liblua5.3.so.0.0.0)
    ==14425==    by 0x5F707D3: sweeplist (in /usr/lib/x86_64-linux-gnu/liblua5.3.so.0.0.0)
    ==14425==    by 0x5F710D0: luaC_freeallobjects (in /usr/lib/x86_64-linux-gnu/liblua5.3.so.0.0.0)
    ==14425==    by 0x5F7715D: close_state (in /usr/lib/x86_64-linux-gnu/liblua5.3.so.0.0.0)
    ==14425==    by 0x443D4C: hlua_deinit (hlua.c:9302)
    ==14425==    by 0x543F88: deinit (haproxy.c:2742)
    ==14425==    by 0x5448E7: deinit_and_exit (haproxy.c:2830)
    ==14425==    by 0x5455D9: init (haproxy.c:2044)

This is due to Lua calling `hlua_alloc()` with `ptr = NULL` and `nsize = 0`.
While `realloc` is supposed to be equivalent `free()` if the size is `0` this
is only required for a non-NULL pointer. Apparently my allocator (or valgrind)
actually allocates a zero size area if the pointer is NULL, possibly taking up
some memory for management structures.

Fix this leak by specifically handling the case where both the pointer and the
size are `0`.

This bug appears to have been introduced with the introduction of the
multi-threaded Lua, thus this fix is specific for 2.4. No backport needed.