login.c: fix minor out of bound strncpy

Signed-off-by: Roosembert Palacios <roosembert.palacios@epfl.ch>

src/login.c

@@ -431,7 +431,7 @@ void shell(struct passwd* pwd)
 		pos = pwd->pw_shell;
 	}
 
-	strncpy(args + 1, pos, 1024);
+	strncpy(args + 1, pos, 1023);
 	execl(pwd->pw_shell, args, NULL);
 }
 
@@ -671,4 +671,4 @@ void auth(
 	{
 		pam_diagnose(ok, buf);
 	}
-}
+}