Fix segfault when printing Bad-echo name after failing to load cfg

This is a weird one: sbbsecho ran right as I was saving msgs.cnf via
SCFG and it seems like it loaded 0-byte msgs.cnf into memory and this
segfault was a result of trying to print sub[INVALID_SUB]->code.

The use of [f]nopen() with proper share perms should have prevented this
(truncated msgs.cnf read) from happening, so there's something else afoot here.

src/sbbs3/sbbsecho.c

@@ -6467,7 +6467,8 @@ int main(int argc, char **argv)
 	}
 
 	if(cfg.badecho >= 0)
-		lprintf(LOG_DEBUG, "Bad-echo area: %s", scfg.sub[cfg.area[cfg.badecho].sub]->code);
+		lprintf(LOG_DEBUG, "Bad-echo area: %s"
+			,cfg.area[cfg.badecho].sub == INVALID_SUB ? "INVALID_SUB" : scfg.sub[cfg.area[cfg.badecho].sub]->code);
 
 	if(cfg.auto_add_subs) {
 		for(unsigned subnum = 0; subnum < scfg.total_subs; subnum++) {