Fix the assertion failure when putting 48-bit number to buffer

Fix the assertion failure when putting 48-bit number to buffer

When putting the 48-bit number into a fixed-size buffer that's exactly 6
bytes, the assertion failure would occur as the 48-bit number is
internally represented as 64-bit number and the code was checking if
there is enough space for `sizeof(val)`.  This causes assertion failure
when otherwise valid TSIG signature has a bad timing information.

Specify the size of the argument explicitly, so the 48-bit number
doesn't require 8-byte long buffer.