Sorry. I found that the number of soft precision bits must be less than 14bits. So I have used 7 for soft precision bits.
Syntax after the modification decoded_data = vitdec(out_interleaved, trellis_type,34,'trunc','soft','7');
Still I am getting error as Number of soft decision bits must be a positive scalar integer.
Note: I am feeding both positive and negative values to the vitdec function.