add missing (std::nothrow)

+ remove unnecessary `.get()` in std::unique_ptr nullptr checks.

Follow up to:
0d1b9941b Fix nothrow new for PrimaryChromaticity parsing

This patch does not address the calls in webm_parser/ where
`(std::nothrow)` is not currently used. Additional work could also be
done to replace `NULL` with `nullptr`.

Change-Id: I85c7cc41ebce5a2533868af4fd957b42ef59cd0c