ps_fdt_read_path error handling inconsistent

[bennoleslie]

ps_fdt_read_path does not clearly document when the state of the cookie and initialization in cases when an error is returned.

This has led to some call-sites checking the error and then calling ps_fdt_cleanup_cookie, and other call-sites not calling ps_fdt_cleanup_cookie.

From examination of the implementation is is clear that ps_fdt_cleanup_cookie should not be called in the cases where an error is returned. So the code that does do this cleanup (such as imx/gpt.c) should be fixed to omit the call to cleanup.
The current behaviour is such that we end up with a double free, which is very difficult to debug.

In addition, to fixing the existing incorrect calls to ps_fdt_cleanup_cookie I'd suggest the following improvements:

• Update documentation of ps_fdt_read_path to make it clear when ps_fdt_cleanup_cookie should be called.
• Update implementation of ps_fdt_read_path so that the cookie output variable is only populated in the case of success (or explicitly populated will NULL on the case of failure). Such a change would make it clearer to callers whether or not to call cleanup, and would hit NULL-pointer check asserts, if ps_fdt_cleanup_cookie is called incorrectly rather than ending up with a double free that just ends up crashing with no visible output.