plug_holes() function
"Plug holes" by combining adjacent empty struct mems. After this function is through, there should not exist one empty struct mem pointing to another empty struct mem. This assumes access to the heap is protected by the calling function already.
Syntax
static void
plug_holes(struct mem *mem);
Arguments
mem
this points to a struct mem which just has been freed @internal this function is only called by mem_free() and mem_trim()