PRE_PROXY change leaks stale Proxy Digest state across proxy-chain boundary

After a Digest-authenticated HTTP proxy transfer, changing only `CURLOPT_PRE_PROXY` on the same libcurl easy handle does not clear stale proxy Digest/auth state. If the new SOCKS pre-proxy resolves the same HTTP proxy hostname to a different proxy endpoint, the second proxy receives a preemptive …

lib/setopt.c:1639: static CURLcode setproxy(...)
lib/setopt.c:1646: changeproxy(data);
lib/setopt.c:1745: return setproxy(data, ptr);