memory: rationalize error handling
memory: rationalize error handling

The code uses a mix of nullptr and bad_alloc. Standardize on nullptr and
only generate bad_alloc at the entry points.

scylladb/seastar,dreamsxin/seastar,dreamsxin/seastar,syuu1228/seastar,cloudius-systems/seastar,syuu1228/seastar,syuu1228/seastar,cloudius-systems/seastar,scylladb/seastar,avikivity/seastar,avikivity/seastar,scylladb/seastar,avikivity/seastar,dreamsxin/seastar,cloudius-systems/seastar