prnatid: Recover from getnatp() failure

cname() calls getnatp(), and returns a null pointer when it fails.
Some systems (GNU, Windows) deal gracefully with printing null
strings, others crash.

Because we keep table EF_NATION entirely in memory, getnatp() should
fail only on invalid country number.

Rewrite prnatid() to catch this error and recover.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>

src/lib/subs/natsub.c

@@ -82,5 +82,9 @@ prnat(struct natstr *np)
 char *
 prnatid(natid cnum)
 {
-    return prbuf("%s (#%d)", cname(cnum), cnum);
+    struct natstr *np = getnatp(cnum);
+
+    if (CANT_HAPPEN(!np))
+	return prbuf("%d (#%d)", cnum, cnum);
+    return prnat(np);
 }