CurlInputPlugin: fix crash after second init call

The second time init was called, http_200_aliases pointed to a freed pointer
and leaded to a SEGFAULT.

src/input/plugins/CurlInputPlugin.cxx

@@ -587,6 +587,7 @@ input_curl_finish(void)
 		});
 
 	curl_slist_free_all(http_200_aliases);
+	http_200_aliases = nullptr;
 
 	curl_global_cleanup();
 }