iconv_open: Fix undefined behaviour

[Bruno Haible]

Shifting an 'unsigned char' value left by 24 bits is undefined behaviour,
if that value happens to be >= 128. (Because the 'unsigned char' value
gets zero-extended to an 'int' first, and left-shift on signed integer types
is undefined if it changes the sign.)


2024-10-04  Bruno Haible  <br...@clisp.org>

        iconv_open: Fix undefined behaviour.
        Reported by Tim Sweet <tswee...@protonmail.com>
        at <https://savannah.gnu.org/bugs/?66289>.
        * lib/iconv.c (utf32be_mbtowc, utf32le_mbtowc): Cast 'unsigned char'
        values to ucs4_t before shifting them to the left.

diff --git a/lib/iconv.c b/lib/iconv.c
index 310f4043eb..f7a67798fb 100644
--- a/lib/iconv.c
+++ b/lib/iconv.c
@@ -195,7 +195,10 @@ utf32be_mbtowc (ucs4_t *pwc, const unsigned char *s, 
size_t n)
 {
   if (n >= 4)
     {
-      ucs4_t wc = (s[0] << 24) + (s[1] << 16) + (s[2] << 8) + s[3];
+      ucs4_t wc =   ((ucs4_t) s[0] << 24)
+                  + ((ucs4_t) s[1] << 16)
+                  + ((ucs4_t) s[2] << 8)
+                  +  (ucs4_t) s[3];
       if (wc < 0x110000 && !(wc >= 0xd800 && wc < 0xe000))
         {
           *pwc = wc;
@@ -237,7 +240,10 @@ utf32le_mbtowc (ucs4_t *pwc, const unsigned char *s, 
size_t n)
 {
   if (n >= 4)
     {
-      ucs4_t wc = s[0] + (s[1] << 8) + (s[2] << 16) + (s[3] << 24);
+      ucs4_t wc =    (ucs4_t) s[0]
+                  + ((ucs4_t) s[1] << 8)
+                  + ((ucs4_t) s[2] << 16)
+                  + ((ucs4_t) s[3] << 24);
       if (wc < 0x110000 && !(wc >= 0xd800 && wc < 0xe000))
         {
           *pwc = wc;