Wrap pgp_uid_t->addr in NONULL(). (closes #3564)

The previous patch introduced the possibility for addr to be null.  Mutt
is surprisingly robust against null strings, but there are a few places
that should be wrapped in NONULL().

gnupgparse.c

@@ -294,7 +294,7 @@ static pgp_key_t parse_pub_line (char *buf, int *is_subkey, pgp_key_t k)
 	if (!is_uid && (*is_subkey && option (OPTPGPIGNORESUB)))
 	  break;
 
-	dprint (2, (debugfile, "user ID: %s\n", p));
+	dprint (2, (debugfile, "user ID: %s\n", NONULL (p)));
 
 	uid = safe_calloc (sizeof (pgp_uid_t), 1);
 	fix_uid (p);

pgpkey.c

@@ -228,7 +228,7 @@ static const char *pgp_entry_fmt (char *dest,
       if (!optional)
       {
 	snprintf (fmt, sizeof (fmt), "%%%ss", prefix);
-	snprintf (dest, destlen, fmt, uid->addr);
+	snprintf (dest, destlen, fmt, NONULL (uid->addr));
       }
       break;
     case 'a':
@@ -586,7 +586,7 @@ static pgp_key_t pgp_select_key (pgp_key_t keys,
 
     case OP_VIEW_ID:
 
-      mutt_message ("%s", KeyTable[menu->current]->addr);
+      mutt_message ("%s", NONULL (KeyTable[menu->current]->addr));
       break;
 
     case OP_GENERIC_SELECT_ENTRY:
@@ -980,7 +980,7 @@ pgp_key_t pgp_getkeybystr (char *p, short abilities, pgp_ring_t keyring)
     for (a = k->address; a; a = a->next)
     {
       dprint (5, (debugfile, "pgp_getkeybystr: matching \"%s\" against key %s, \"%s\": ",
-		  p, pgp_keyid (k), a->addr));
+		  p, pgp_keyid (k), NONULL (a->addr)));
       if (!*p || mutt_strcasecmp (p, pgp_keyid (k)) == 0 ||
 	  (!mutt_strncasecmp (p, "0x", 2) && !mutt_strcasecmp (p + 2, pgp_keyid (k))) ||
 	  (option (OPTPGPLONGIDS) && !mutt_strncasecmp (p, "0x", 2) &&