fix compile_rx_or_error

fix compile_rx_or_error

compile_rx_or_error looks like a constructor, but it can return NULL.
This patch changes it to remove the NULL return, making it work
like any other cleanup constructor.

This is a stylistic patch but I think it is also better for code to
follow the normal conventions.

	* probe.c (collect_probes): Check arguments for NULL before
	calling compile_rx_or_error.
	* utils.c (compile_rx_or_error): Require 'rx' to be non-NULL.
	Remove NULL return.