src: Avoid copy in evbuffer_pullup()

Previously, we use evbuffer_pullup(buf, -1) to linearize the memory
region and it may cause buffer copy.  To avoid this, we use the return
value of evbuffer_get_contiguous_space() as 2nd parameter.  According
to the libevent manual, by doing so evbuffer_pullup() will not copy or
modify any data in evbuffer.