Fix NULL pointer dereference in vPortGetHeapStats

Fix NULL pointer dereference in vPortGetHeapStats

When the heap is exhausted (no free block), start and end markers are
the only blocks present in the free block list:

     +---------------+     +-----------> NULL
     |               |     |
     |               V     |
+ ----- +            + ----- +
|   |   |            |   |   |
|   |   |            |   |   |
+ ----- +            + ----- +
  xStart               pxEnd

The code block which traverses the list of free blocks to calculate heap
stats used a do..while loop that moved past the end marker when the heap
had no free block resulting in a NULL pointer dereference. This commit
changes the do..while loop to while loop thereby ensuring that we never
move past the end marker.

This was reported here - https://github.com/FreeRTOS/FreeRTOS-Kernel/issues/534

Signed-off-by: Gaurav Aggarwal <aggarg@amazon.com>