fix: possible segfault in 64 bit deserialize_safe

[Dr-Emann]

The added test fails in ASAN without this change with an out-of-bounds read when art_insert_at computes a common prefix of 6 (all 48 bits), and tries to read the first non-matching byte of the key at

CRoaring/src/art/art.c

Lines 1172 to 1175 in 9526ecf

	new_node = art_node_insert_leaf((art_inner_node_t *)new_node,
	leaf->key[depth + common_prefix], leaf);
	new_node = art_node_insert_leaf((art_inner_node_t *)new_node,
	key[depth + common_prefix], new_leaf);

[lemire]

Excellent.