fix build errors with -DNDEBUG

[Martin Sebor]

Building the top of trunk with the -DNDEBUG option fails due to
local variables that are assigned to but only used in assertions,
and due to one instance where a variable is reported as possibly
used before initialized.

The attached patch suppresses these errors and allows the top of
trunk to build. Verified on powerpc64le.

Is it still time to commit this on trunk or should it wait until
after the release?

Martin
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