dr_raw_tls_calloc does not init slots to 0

**Describe the bug**

I started implementing a new tool in DynamoRIO today, and assumed that dr_raw_tls_calloc will init slots to 0 in accordance to the docs pasted here:

> These slots will be initialized to 0 for each new thread.

However, this does not seem to be the case. Is this a bug in DR or a mistake in the docs (or maybe a possible bug in my test case)

**To Reproduce**

Test case:

```
#include "drmgr.h"
#include "drreg.h"
#include "drutil.h"
#include "utils.h"

reg_id_t tls_raw_reg;
uint tls_raw_base;
static void event_exit(void);

static void **get_tls_addr(int slot_idx) {

byte *seg_base = dr_get_dr_segment_base(tls_raw_reg);
byte *addr = (byte *) ( seg_base + tls_raw_base + slot_idx * sizeof(void *));
return *((void **) addr);
}

static void
event_thread_init(void *drcontext)
{
DR_ASSERT(get_tls_addr(0) == NULL);
DR_ASSERT(get_tls_addr(1) == NULL);
DR_ASSERT(get_tls_addr(2) == NULL);
DR_ASSERT(get_tls_addr(3) == NULL);
}

DR_EXPORT void dr_client_main(client_id_t id, int argc, const char *argv[]) {

drmgr_init();
drutil_init();

dr_register_exit_event(event_exit);

dr_raw_tls_calloc(&(tls_raw_reg), &(tls_raw_base), 4, 0);

if (!drmgr_register_thread_init_event(event_thread_init))
DR_ASSERT(false);
}

static void event_exit() {

if (!drmgr_unregister_thread_init_event(event_thread_init))
DR_ASSERT(false);

dr_raw_tls_cfree(tls_raw_base, 4);

drutil_exit();
drmgr_exit();
}

```
**Expected behavior**

Slots should be zero at thread init events.

**Screenshots or Pasted Text**

```

ASSERT FAILURE: /home/john/main_dr/dynamorio/api/samples/testcase.c:25: get_tls_addr(1) == ((void *)0) ()

```

**Versions**

Current head commit, on 32-bit Linux

**Additional context**

I looked at how current DR tools make use of dr_raw_tls_calloc. Dr Memory and drreg both write to the slots before use.

Assign this issue to me. I would imagine we are missing a memset somewhere/or the doc is incorrect due to some hard limitation which I am not aware of. Any pointer where I should look in DR to fix this would also help me.