Fix warnings treated as errors with Werror

This fixes warnings caused by:
 * setting unused variables.
 * not checking return values from functions with the __must_check
   attribute.

Bug: 211637748
Signed-off-by: Will McVicker <willmcvicker@google.com>
Change-Id: I5063cf465d224451763a5b6098ee0ac2d859d7d4