The json.encode() used to cause a segfault in case of recursive
table:
  tbl = {}
  tbl[1] = tbl
  json.encode(tbl)

Library doesn't test whether given object on Lua stack parsed
earlier, because it performs a lightweight in-depth traverse
of Lua stack. However it must stop when encode_max_depth is
reached (by design).

Tarantool's lua_cjson implementation has a bug introduced during
porting original library: it doesn't handle some corner cases:
entering into a map correctly increases a current depth, while
entering into an array didn't. This patch adopts author's
approach to check encode_max_depth limit. Thanks to handling this
constraint correctly the segfault no longer occurs.

Closes #4366

(cherry picked from commit d2641e66e65ab57317b8e70bb7115517ec81238e)