20251029-NullPointerArithm-fixes

[douzzer]

src/internal.c: in HashOutput(), check for null output pointer;

examples/pem/pem.c: in main(), add missing check that ret == 0 in _DER_TO_PEM code path.

tested with

wolfssl-multi-test.sh ...
check-source-text
clang-tidy-Customer-CFG-7
clang-tidy-defaults
clang-tidy-intmath
clang-tidy-fips-140-3-defaults
clang-tidy-fips-140-3-pilot-defaults
clang-tidy-fips-140-3-dev-defaults
clang-tidy-fips-140-3-dev-defaults-no-sha-1
clang-tidy-fips-140-3-v6-defaults
clang-tidy-all-crypto-no-sha-1