os/mm/umm_heap: Fix memory leak in umm_free
os/mm/umm_heap: Fix memory leak in umm_free

umm_free can fail under the below scenario:
- Two tasks are running
- Task 1 exits. Task 2 is now active
- The task 1 clean up code calls kumm_free to free its stack
- umm_free() tries to get heap using mm_get_heapindex() API
- Since, task 2 is now active, this API returns -1

The issue is fixed by replacing mm_get_heapindex() with mm_get_heap()
which has the logic to search all the app heaps to find a match for
the given address.

Signed-off-by: Kishore SN <33058dcca7ce956a91a61ef5ae8fb790ece75b5f@samsung.com>

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