http.c: Fix problem with repeated calls of http_init

Calling http_init after calling http_cleanup causes a segfault.  This
is due to the pragma_header curl_slist being freed but not being set
to NULL.  The subsequent call to http_init tries to setup the slist
again, but it now points to an invalid memory location.
Signed-off-by: NJulian Phillips <julian@quantumfyre.co.uk>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

http.c

@@ -300,6 +300,7 @@ void http_cleanup(void)
 	curl_global_cleanup();
 
 	curl_slist_free_all(pragma_header);
+        pragma_header = NULL;
 }
 
 struct active_request_slot *get_active_slot(void)