When splitting a memory block, check if the next block is free.
If it is, then just extend it upwards instead of creating a new block.
This fixes a bug where when shrinking existing allocations would result in irreversible free space fragmentation.
When testing on the host, test all the poisoning configurations.
During a call to multi_heap_malloc(), if both these conditions were true:
- That heap only has one block large enough for the allocation
(this is always the case if the heap is unfragmented).
- Another allocation is simultaneously occurring in the same heap.
... multi_heap_malloc() could incorrectly return NULL.
This caused IDF heap_caps_malloc() and malloc() to also fail, particularly
often if only one or two heaps had space for the allocation (otherwise
heap_caps_malloc() fails over to the next heap).
New multi_heap code has proven effective at aborting when buffer overruns occur,
but it's currently hard to debug the stack traces from these failures.
