Make heap small chunk size setting logic more precise/correct

Prior to this commit, the heap small chunk size setting logic would
bitwise-or in the size into the appropriate bits of the small chunk
but this is technically incorrect as it doesn't clear out the bits
first.

This commit changes the logic to ensure the relevant bits are cleared
first before the new size is bitwise-or'd in.

src/libponyrt/mem/heap.c

@@ -212,7 +212,7 @@ static void set_small_chunk_size(small_chunk_t* chunk, size_t size)
 
   // left shift size to get bits in the right spot for OR'ing into `chunk->m`
   size = size << SMALL_CHUNK_SIZECLASS_SHIFT;
-  ((chunk_t*)chunk)->m = (char*)((uintptr_t)((chunk_t*)chunk)->m | (size & SMALL_CHUNK_SIZECLASS_BITMASK));
+  ((chunk_t*)chunk)->m = (char*)(((uintptr_t)((chunk_t*)chunk)->m & ~SMALL_CHUNK_SIZECLASS_BITMASK) | (size & SMALL_CHUNK_SIZECLASS_BITMASK));
 }
 
 static bool get_chunk_needs_clearing(chunk_t* chunk)