[Tensor Decoder] Security: fixed not null termination of strncpy(3)

[Tensor Decoder] Security: fixed not null termination of strncpy(3)

Fixed issue #690.

This commit is to fix not null termination issue in strncpy library call
in the tensor decoder element.

strncpy() supposedly protects from buffer overflows. But if it prevents
an overflow without null terminating, in all likelyhood a subsequent
string operation is going to overflow.

**Change proposed in this PR:**
* Version 3:
  1. Commented security, gstreamer structure, and Svace (Won't fix)
  2. Keep Gstreamer (gstbasetextoverlay.c) policy

* Version 2:
  1. Replaced strncpy with memcpy.
  2. Added annotation.

* Version 1:
  1. Fixed not null termination issue of strncpy() by using memset().

Signed-off-by: Geunsik Lim <geunsik.lim@samsung.com>