popmaildir: placate gcc8 "warning: 'strncpy' output truncated"

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

mailutils/popmaildir.c

@@ -265,7 +265,7 @@ int popmaildir_main(int argc UNUSED_PARAM, char **argv)
 
 		// atomically move message to ./new/
 		target = xstrdup(filename);
-		strncpy(target, "new", 3);
+		memcpy(target, "new", 3);
 		// ... or just stop receiving on failure
 		if (rename_or_warn(filename, target))
 			break;