Preserve the contents of socket buffer on realloc
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.
Closes #2917 (closed)
Edited by Ondřej Surý