pn_data_t initialization lead to low performance

Recently I update qpid-proton from 0.28.0 to 0.30.0 version, and find massive produce performance fallen by 80%。

Finally，I find this is caused by pn_data_t initialization。 In 0.28.0 version, it's like 
data->buf = pn_buffer(64);
and in In 0.30.0 version, it's like 
data->buf = NULL;
 
And when data->buf is initialized by NULL,  in the following call flow, 
pn_data_put_binary->pni_data_intern_node->pn_buffer_memory->pn_buffer_defrag->pn_buffer_rotate, 
buf->start is more likely not equal to 0, this is leading to buffer rotate, and make the performance lower.