diag: Use correct size while reallocating for hdlc encoding

Possibility of mismatch with realloc size against the buf size
can cause out of bound scenario. Prevent the same using
proper size for reallocation of hdlc encoding.

Change-Id: I0c0f822bc591af69b009967c89d378ac70f79678
Signed-off-by: Manoj Prabhu B <quic_bmanoj@quicinc.com>