gRPC: clearing buffers in ngx_http_grpc_get_buf().

gRPC: clearing buffers in ngx_http_grpc_get_buf().

We copy input buffers to our buffers, so various flags might be
unexpectedly set in buffers returned by ngx_chain_get_free_buf().

In particular, the b->in_file flag might be set when the body was
written to a file in a different context.  With sendfile enabled this
in turn might result in protocol corruption if such a buffer was reused
for a control frame.

Make sure to clear buffers and set only fields we really need to be set.