Fix uses of ProtobufCBufferSimple that are 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. Various instances of ProtobufCBufferSimple
in nmsg were allocated on the stack without zeroing the entire object,
which resulted in protobuf-c's memory allocation function dereferencing
a garbage pointer.

Always initialize ProtobufCBufferSimple objects.