http: die on curl_easy_duphandle failure in get_active_slot

http: die on curl_easy_duphandle failure in get_active_slot

get_active_slot() duplicates the default curl handle via
curl_easy_duphandle() to create a per-slot session handle. The
return value is stored directly in slot->curl without checking
for NULL. curl_easy_duphandle() can return NULL when memory
allocation fails internally, and the libcurl documentation
explicitly states this possibility.

When this happens, slot->curl is NULL and the very next operation
(curl_easy_setopt on line 1632 for CURLOPT_COOKIEFILE) passes
NULL as the curl handle, which is undefined behavior in libcurl
and typically crashes.

Every HTTP operation in git goes through get_active_slot(), so
this affects all remote-https, remote-http, and HTTP-based
operations (clone, fetch, push over HTTP, bundle-uri downloads).

Add a NULL check and die() with a clear message. There is no
reasonable recovery from a failed handle duplication: the process
is out of memory and cannot perform any HTTP operation.

Pointed out by Coverity.

Assisted-by: Claude Opus 4.6
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>