Add SUBOBJECT_INITIALIZER and use it.

Add SUBOBJECT_INITIALIZER and use it.

Just as we have OD_ENTRY_INITIALIZER, add SUBOBJECT_INITIALIZER, and use
it rather than memset().

Whether removing initializer warnings is a Good Thing is subject to
debate; remove a comment that implies it's been deemed a Good Thing.

Change-Id: Ife658d8bb1d4868789ca3b929aff6e4fccecb430
Reviewed-on: https://code.wireshark.org/review/21892
Reviewed-by: Guy Harris <guy@alum.mit.edu>