rrd_client: avoid invalid read

rrd_client: avoid invalid read

In rrd_client_last and rrd_client_first, the response value is in the
message prior to any additional lines.  Unfortunately, the buffer that
contains this data is on the stack for response_read and is no longer valid
by the time the caller sees it.

This change strdups the message line in response_read and subsequently frees
it in response_free.

Resolves issue #1047.