Fix instances of pointer addition with the NULL pointer

Fix instances of pointer addition with the NULL pointer

ubsan found undefined pointer addtions in
crypto/bio/bss_mem.c (mem_ctrl),
crypto/pem/pem_lib.c (PEM_read_bio_ex),
test/testutil/format_output.c (test_fail_string_common,
test_fail_memory_common).

Mostly a straight back-port-of: a07dc81

Additionally enable the ubsan run-checker, to prevent regressions.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Nicola Tuveri <nic.tuv@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/16423)