Check for null bytes in filenames in 'expand-file-name' (bug#49723)

* src/fileio.c (expand-file-name): Check for null bytes for both
NAME and DEFAULT-DIRECTORY arguments.  Also check for null bytes
in buffer-local default-directory, assuming it is used.
* src/coding.c (encode_file_name): Use CHECK_STRING_NULL_BYTES.
* src/lisp.h (CHECK_STRING_NULL_BYTES): Add function for checking
for null bytes in Lisp strings.
* test/src/fileio-tests.el (fileio-test--expand-file-name-null-bytes):
Add test for new changes to expand-file-name.
* etc/NEWS: Announce changes.

1 files changed, 5 insertions, 1 deletions

diff --git a/src/fileio.c b/src/fileio.c
index 0db8ed793b3..3c13d3fe416 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -945,6 +945,7 @@ the root directory.  */)
   USE_SAFE_ALLOCA;
 
   CHECK_STRING (name);
+  CHECK_STRING_NULL_BYTES (name);
 
   /* If the file name has special constructs in it,
      call the corresponding file name handler.  */
@@ -993,7 +994,10 @@ the root directory.  */)
       if (STRINGP (dir))
         {
           if (file_name_absolute_no_tilde_p (dir))
-            default_directory = dir;
+            {
+              CHECK_STRING_NULL_BYTES (dir);
+              default_directory = dir;
+            }
           else
             {
               Lisp_Object absdir