Suppress possibly-uninitialized-variable warnings from gcc 4.5.

Suppress possibly-uninitialized-variable warnings from gcc 4.5.

It appears that gcc 4.5 can issue such warnings for whole structs, not
just scalar variables as in the past.  Refactor some pg_dump code slightly
so that the OutputContext local variables are always initialized, even
if they won't be used.  It's cheap enough to not be worth worrying about.