Interworking: Check NULL string to avoid compiler warning

Interworking: Check NULL string to avoid compiler warning

Fix warning by gcc 9.2.1.

interworking.c: In function ‘interworking_home_sp_cred’:
interworking.c:2263:3: warning: ‘%s’ directive argument is null [-Wformat-overflow=]
 2263 |   wpa_msg(wpa_s, MSG_DEBUG,
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~
 2264 |    "Interworking: Search for match with SIM/USIM domain %s",
      |    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 2265 |    realm);
      |    ~~~~~~

Signed-off-by: Masashi Honma <masashi.honma@gmail.com>