lib/chkname.c: An object cannot expand further than the universe in which it resides

If you want a larger object, you'll have to wait at least until the
universe expands so much.

If an implementation doesn't recognize its own limitations, its a bug,
not a feature.

Closes: <https://github.com/shadow-maint/shadow/issues/1052>
Cc: Tobias Stoeckmann <tobias@stoeckmann.org>
Signed-off-by: Alejandro Colomar <alx@kernel.org>

lib/chkname.c

@@ -24,6 +24,8 @@
 #include <limits.h>
 #include <stdbool.h>
 #include <stddef.h>
+#include <stdint.h>
+#include <sys/param.h>
 #include <unistd.h>
 
 #include "defines.h"
@@ -43,7 +45,7 @@ login_name_max_size(void)
 	if (conf == -1 && errno != 0)
 		return LOGIN_NAME_MAX;
 
-	return conf;
+	return MIN(conf, PTRDIFF_MAX);
 }