[NFC] ASSERT_EQ before accessing items in containers

As discussed offline, something different from `EXPECT_EQ` should be
used to check if the container contains enough items before accessing
them so that other tests can still be run even if the assertion fails as
opposed to having `EXPECT_EQ` failing and then aborting the run due to
the errors caused by out-of-bounds memory access.

Reviewed by: ilya-biryukov

Differential Revision: https://reviews.llvm.org/D70528