Add LCOV_EXCL* markers to crypto.c and crypto_s2k.c

Add LCOV_EXCL* markers to crypto.c and crypto_s2k.c

This marks some lines as unreachable by the unit tests, and as
therefore excluded from test coverage.

(Note: This convention is only for lines that are absolutely
unreachable.  Don't use it anywhere you wouldn't add a
tor_fragile_assert().)