Refactor pgp_key_t methods for noexcept and fix few coverity issues. #3650

Job	Run time
cmake-offline-googletest-src	4m 27s
ubuntu-latest [CC gcc; botan; shared libs on; GnuPG stable]	4m 43s
ubuntu-latest [CC clang; botan; shared libs on; GnuPG stable]	4m 58s
ubuntu-latest [CC gcc; openssl; shared libs on; GnuPG stable]	3m 56s
cmake-offline-googletest	4m 17s
ubuntu-latest [CC clang; openssl; shared libs on; GnuPG stable]	4m 22s
ubuntu-20.04 [CC gcc; openssl; shared libs on; GnuPG stable]	3m 54s
package-source	29s
system-sexpp, sexpp shared libs on, rnp shared libs on	3m 11s
ubuntu-latest [CC gcc; botan; shared libs off; GnuPG stable]	4m 32s
system-sexpp, sexpp shared libs on, rnp shared libs off	2m 44s
system-sexpp, sexpp shared libs off, rnp shared libs on	2m 51s
system-sexpp, sexpp shared libs off, rnp shared libs off	4m 0s
package	1m 9s
debian-tests	33s
	50m 6s

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