curl: HTTP/2 PUSH_PROMISE header loss on OOM bypasses scheme validation (regression of 2e8c922a89)

Summary: In lib/http2.c:1490, when curlmaprintf fails due to memory pressure, the push promise header is silently dropped but the callback returns success. If the lost header is the :scheme pseudo-header, the security check at line 733 that blocks HTTPS pushes over insecure connections is skipped...