health: Check return value of LinkedCallback::Make.

If LinkedCallback::Make returns nullptr, do not put it
in Health::callback_. Otherwise, OnHealthInfoChanged
crashes later because the linked callback objects are
not null checked before accessing.

Test: android.hardware.health-service.aidl_fuzzer (with
  a special corpus)
Fixes: 289599278
Change-Id: I8bad41dbcfbefeb54744059baffd4eef1ae7ec42