rethook: fix a potential memleak in rethook_alloc()

In the Linux kernel, the following vulnerability has been resolved: rethook: fix a potential memleak in rethook_alloc() In rethook_alloc(), the variable rh is not freed or passed out if handler is NULL, which could lead to a memleak, fix it. [Masami: Add "rethook:" tag to the title.] Acke-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>