Fix possible error that cause double free

Fix possible error that cause double free

There is an occasional error from unittest_tizen_capi.

```bash
[ RUN      ] nntrainer_capi_nnmodel.train_with_file_01_p
training
data_buffer made
data_buffer made done
size: 2
data_buffer clear
data_buffer end
double free or corruption (out)
Aborted (core dumped)
```

**Changes proposed in this PR:**
join data buffer threads first before clearing vectors that are
used in the target function to (possibly) prevent double free

**Self evaluation:**
1. Build test: [X]Passed [ ]Failed [ ]Skipped
2. Run test: [X]Passed [ ]Failed [ ]Skipped

Signed-off-by: Jihoon Lee <jhoon.it.lee@samsung.com>