fixup! add support for disabling SSL revocation checks in cURL

This maintainer thought that it was discussed that you cannot `strcmp()`
blindly without checking for NULL first, but subsequently forgot to
check that the fix had made it into the final version of the PR... :-(

The problem is that this crashes any fetch/push/clone/ls-remote
operation *unless* http.sslBackend is specified explicitly.

This probably fixes the root cause behind
#1474.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

http.c

@@ -753,7 +753,8 @@ static CURL *get_curl_handle(void)
 	}
 #endif
 
-	if (!strcmp("schannel", http_ssl_backend) && !http_schannel_check_revoke) {
+	if (http_ssl_backend && !strcmp("schannel", http_ssl_backend) &&
+	    !http_schannel_check_revoke) {
 #if LIBCURL_VERSION_NUM >= 0x074400
 		curl_easy_setopt(result, CURLOPT_SSL_OPTIONS, CURLSSLOPT_NO_REVOKE);
 #else