input/curl: fixed endless loop during buffering

When the connection is lost while buffering, the CURL input plugin may
enter an endless loop, because it does not check the EOF condition.
This patch makes fill_buffer() return success only if there's at least
one buffer, which is enough of a check.x

src/input/curl_input_plugin.c

@@ -315,7 +315,7 @@ fill_buffer(struct input_stream *is)
 			return false;
 	}
 
-	return true;
+	return !g_queue_is_empty(c->buffers);
 }
 
 /**