libpagemap: fix memory leak in pm_process_destroy()

procrank/librank not impacted because they end quickly. But other
programs that use libpagemap and last for long time can easily see this
memory leak.

Change-Id: I8c9e9444555bef9145c9d89850987a29f15a9b3b
Signed-off-by: Carton He <carton.he@marvell.com>