curl: Keep *socket until the end of curl_sock_cb()

curl: Keep *socket until the end of curl_sock_cb()

This does not really change anything, but it makes the code a bit easier
to follow once we use @socket as the opaque pointer for
aio_set_fd_handler().

Cc: qemu-stable@nongnu.org
Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20190910124136.10565-3-mreitz@redhat.com
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
(cherry picked from commit 007f339b1099af46a008dac438ca0943e31dba72)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>