dnstap: Fix ProtobufCBufferSimple usage that is incorrect as of protobuf-c 1.0.0

protobuf-c 1.0.0 added a new 'allocator' field to ProtobufCBufferSimple
that controls memory allocation, which must be NULL in order to request
the default system allocator. Allocating ProtobufCBufferSimple objects
on the stack without zeroing the entire object can result in
protobuf-c's memory allocation functions dereferencing a garbage
pointer.

Signed-off-by: Jan Vcelak <jan.vcelak@nic.cz>