core,shared: add a couple of asserts

core,shared: add a couple of asserts

To appease the LTO overlords:

In file included from ../../../../src/basic/macro.h:446,
                 from ../../../../src/shared/blockdev-util.h:8,
                 from ../../../../src/shared/creds-util.c:11:
../../../../src/shared/creds-util.c: In function 'get_credential_host_secret':
../../../../src/shared/creds-util.c:379:52: error: '%s' directive argument is null [-Werror=format-overflow=]
  379 |                                 log_debug_errno(r, "Credential secret %s/%s appeared while we were creating it, rereading.",
      |                                                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../../../../src/basic/log.h:214:86: note: in definition of macro 'log_full_errno_zerook'
  214 |                         ? log_internal(_level, _e, PROJECT_FILE, __LINE__, __func__, __VA_ARGS__) \
      |                                                                                      ^~~~~~~~~~~
../../../../src/basic/log.h:250:41: note: in expansion of macro 'log_full_errno'
  250 | #define log_debug_errno(error, ...)     log_full_errno(LOG_DEBUG,   error, __VA_ARGS__)
      |                                         ^~~~~~~~~~~~~~
../../../../src/shared/creds-util.c:379:33: note: in expansion of macro 'log_debug_errno'
  379 |                                 log_debug_errno(r, "Credential secret %s/%s appeared while we were creating it, rereading.",
      |                                 ^~~~~~~~~~~~~~~
../../../../src/shared/creds-util.c:379:74: note: format string is defined here
  379 |                                 log_debug_errno(r, "Credential secret %s/%s appeared while we were creating it, rereading.",
      |                                                                          ^~
cc1: some warnings being treated as errors

../../../../src/core/transaction.c: In function 'transaction_verify_order_one':
../../../../src/core/transaction.c:338:38: error: '%s' directive argument is null [-Werror=format-overflow=]
  338 |                 sprintf(ans + size, "%s%s", unit_log_field, *unit_id);
      |                                      ^~
cc1: some warnings being treated as errors