http: always remove curl easy from curlm session on release

We must call curl_multi_remove_handle when releasing the slot to
prevent subsequent calls to curl_multi_add_handle from failing
with CURLM_ADDED_ALREADY (in curl 7.32.1+; older versions
returned CURLM_BAD_EASY_HANDLE)

Signed-off-by: Eric Wong <e@80x24.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: Eric Wong

http.c

@@ -855,11 +855,13 @@ void run_active_slot(struct active_request_slot *slot)
 static void release_active_slot(struct active_request_slot *slot)
 {
 	closedown_active_slot(slot);
-	if (slot->curl && curl_session_count > min_curl_sessions) {
+	if (slot->curl) {
 		xmulti_remove_handle(slot);
-		curl_easy_cleanup(slot->curl);
-		slot->curl = NULL;
-		curl_session_count--;
+		if (curl_session_count > min_curl_sessions) {
+			curl_easy_cleanup(slot->curl);
+			slot->curl = NULL;
+			curl_session_count--;
+		}
 	}
 #ifdef USE_CURL_MULTI
 	fill_active_slots();