[glibc/azanella/ubsan-undef] libio: Fix UB __libio_codecvt_length

[Adhemerval Zanella]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=49431f65444e82c01bd9a5301bdd1bb61169c922

commit 49431f65444e82c01bd9a5301bdd1bb61169c922
Author: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Date:   Tue Apr 22 09:02:54 2025 -0300

    libio: Fix UB __libio_codecvt_length
    
    To avoid a 0 size VLA.

Diff:
---
 libio/iofwide.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/libio/iofwide.c b/libio/iofwide.c
index c00b83c703..e9d6e51ac7 100644
--- a/libio/iofwide.c
+++ b/libio/iofwide.c
@@ -228,6 +228,9 @@ __libio_codecvt_length (struct _IO_codecvt *codecvt, __mbstate_t *statep,
 			const char *from_start, const char *from_end,
 			size_t max)
 {
+  if (max == 0)
+    return 0;
+
   int result;
   const unsigned char *cp = (const unsigned char *) from_start;
   wchar_t to_buf[max];