block/curl: Fix wrong free statement

Report from smatch:
block/curl.c:546 curl_close(21) info: redundant null check on s->url calling free()

The check was redundant, and free was also wrong because the memory
was allocated using g_strdup.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
