Fix mp_tuple_assert function

At the moment mp_tuple_assert() function from src/box/tuple.h seems to be incorrect:

static inline void
mp_tuple_assert(const char *tuple, const char *tuple_end)
{
	assert(mp_typeof(*tuple) == MP_ARRAY);
#ifndef NDEBUG
	mp_next(&tuple);
#endif
	assert(tuple == tuple_end);
	(void) tuple;
	(void) tuple_end;
}

It seems that we need to keep mp_next and assert(tuple == tuple_end) under the same preprocessing:

static inline void
mp_tuple_assert(const char *tuple, const char *tuple_end)
{
	assert(mp_typeof(*tuple) == MP_ARRAY);
#ifndef NDEBUG
	mp_next(&tuple);
	assert(tuple == tuple_end);
#endif
	(void) tuple;
	(void) tuple_end;
}

Everything should break on non-debug builds with assertions (we are lucky that we don't have such).