Avoid underflow on an unsigned int when computering bg overrun

A computation for when a block group overruns the end of an image
was consistently storing a negative number in an unsigned var,
and then checking for >0, which is always true for negative numbers.
So first check if the number will be positive before computing it.

Change-Id: Ic8cff8f9ab9f4ea8c5a4dc42143c2430fa87ba12
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>