[Demux] Fix test break when performance option=1

[Demux] Fix test break when performance option=1

occur segmentation error when testing
```bash
nnstreamer/tests $ ./testAll.sh 1
0:00:00.025194169 21004 0x7f47b0002d90 DEBUG            tensordemux gsttensordemux.c:437:gst_tensor_demux_chain:<demux>  Number or Tensors: 1
0:00:00.025209492 21004 0x7f47b0002d90 DEBUG            tensordemux gsttensordemux.c:307:gst_get_tensor_pad:<demux> createing pad: 0(0th)

****** skip ********

0:00:00.008473223 21133      0x1b51e80 DEBUG            tensordemux gsttensordemux.c:488:gst_tensor_demux_chain:<demux> pushing buffer with timestamp 99:99:99.999999999
Caught SIGSEGV
Spinning.  Please run 'gdb gst-launch-1.0 21170' to continue debugging, Ctrl-C to quit, or Ctrl-\ to dump core.
```

I guess that cause is to typecast integer pointer to char pointer without converting.
This is temporary fix.

Signed-off-by: sewon.oh <sewon.oh@samsung.com>