sgsnemu: Fix null-pointer format string argument

Modern gcc-9.2.1 actually fails like this with --enable-werror active:

In file included from sgsnemu.c:52:
In function ‘process_options’,
    inlined from ‘main’ at sgsnemu.c:1557:6:
../lib/syserr.h:31:3: error: ‘%s’ directive argument is null [-Werror=format-overflow=]
   31 |   logp2(sub, pri, __FILE__, __LINE__, 0,   \
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   32 |    fmt "\n", ##args);    \
      |    ~~~~~~~~~~~~~~~~~
sgsnemu.c:435:3: note: in expansion of macro ‘SYS_ERR’
  435 |   SYS_ERR(DSGSN, LOGL_ERROR, 0,
      |   ^~~~~~~
sgsnemu.c: In function ‘main’:
sgsnemu.c:436:42: note: format string is defined here
  436 |    "Listening address must be specified: %s!",
      |                                          ^~

It is correct: We are dereferencing args_info.listen_addr in a
branch which explicitly checks if that is NULL beforehand :/

Change-Id: I417f447f821d396aa92049e0a791121240f1cf44

sgsnemu/sgsnemu.c

@@ -433,8 +433,7 @@ static int process_options(int argc, char **argv)
 		}
 	} else {
 		SYS_ERR(DSGSN, LOGL_ERROR, 0,
-			"Listening address must be specified: %s!",
-			args_info.listen_arg);
+			"Listening address must be specified!");
 		return -1;
 	}