curl: Check curl_multi_add_handle()'s return code

curl: Check curl_multi_add_handle()'s return code

If we had done that all along, debugging would have been much simpler.
(Also, I/O errors are better than hangs.)

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20190910124136.10565-8-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 c34dc07f9f01cf686e512f939aece744723072cd)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>