test: add coverage for untested allocator error paths

[jvoisin]

Add three small-allocation test cases and wire them into the Makefile and test_smc.py harness:

• invalid_free_aligned_sized_small: free_aligned_sized() with a non-power-of-two alignment, covering the "invalid sized deallocation alignment (small)" fatal path.
• invalid_malloc_object_size_small_canary: malloc_object_size() queried at an offset past the usable region, covering the "invalid malloc_object_size (canary)" fatal path.
• read_canary_after_small: revive the orphaned test that verifies the slack past a small request stays zeroed across alloc/free cycles; fix the NULL check that ran after the memset and tidy indentation.