Rework platform_buffer_create() to take platform_heap_id hid as arg. Fix (minor) latent bug in this routine. #507

platform_buffer_create() currently takes platform_heap_handle UNUSED_PARAM(heap_handle) as the arg to specify the "heap"-handle from which a new mmap()-ed buffer will be created.

Currently, in Splinter, heap_id is NULL, and we fall-back to using malloc(). The heap is used to allocte buffer_handle *bh structure.

Right now, things work ok. In a future dev effort, we are looking to support alternate memory handles, say, a shared-memory segment from which stuff like bh can be allocated. The platform_buffer_create() uses platform_get_heap_id() to access the heap-ID to allocate from (and that's anyway NULL right now).

This issue is being raised to make couple of small changes:

1. Change the interface of platform_buffer_create() to take heap_id as its argument. In future, this will make it easier to drop-in support for other sources of heap-memory given by the heap-ID.
2. There is a minor bug in the current code. This function could access a NULL bh if ever TYPED_MALLOC() were to fail. This code chunk will seg-fault:

 60 platform_buffer_create(size_t               length,
 61                        platform_heap_handle UNUSED_PARAM(heap_handle),
 62                        platform_module_id   UNUSED_PARAM(module_id))
 63 {
 64    buffer_handle *bh = TYPED_MALLOC(platform_get_heap_id(), bh);
 65
 66    if (bh != NULL) {
[...]
 91    bh->length = length;
 92    return bh;