On TCPDNS/TLSDNS read callback, the socket buffer could be reallocated
if the received contents would be larger than the buffer. The existing
code would not preserve the contents of the existing buffer which lead
to the loss of the already received data.
This commit changes the isc_mem_put()+isc_mem_get() with isc_mem_reget()
to preserve the existing contents of the socket buffer.
8248da3b83