Fix a wrong calculation of buffer size to make coap options

Fix a wrong calculation of buffer size to make coap options

A 'buflen' variable produced by coap_split_query has a meaning of
remained buffer size after filling a set of coap query options to pBuf which
has CA_MAX_URI_LENGTH(512) size.
It means option data is filled upto pBuf + (CA_MAX_URI_LENGTH - buflen).
Thus, accessing pBuf can reach to the above location.

Change-Id: I1320531730800a57e78e4bc56552c51525903468
Signed-off-by: Jihun Ha <jihun.ha@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/18363
Tested-by: jenkins-iotivity <jenkins@iotivity.org>
Reviewed-by: Dan Mihai <Daniel.Mihai@microsoft.com>