Fix bug #18516 with SIGSEGV in expand-file-name.

 src/fileio.c (Fexpand_file_name) [DOS_NT]: Make sure newdirlim is
 always set to a valid value.  Make sure the size passed to alloca
 is always positive.

1 files changed, 3 insertions, 0 deletions

diff --git a/src/fileio.c b/src/fileio.c
index 80905c63d3a..13e2c889020 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -1237,6 +1237,8 @@ filesystem tree, not (expand-file-name ".."  dirname).  */)
               newdirlim = adir + SBYTES (tem);
               memcpy (adir, SSDATA (tem), SBYTES (tem) + 1);
             }
+          else
+            newdirlim = adir + strlen (adir);
         }
       if (!adir)
         {
@@ -1383,6 +1385,7 @@ filesystem tree, not (expand-file-name ".."  dirname).  */)
 
   /* Now concatenate the directory and name to new space in the stack frame.  */
   tlen = length + file_name_as_directory_slop + (nmlim - nm) + 1;
+  eassert (tlen > file_name_as_directory_slop + 1);
 #ifdef DOS_NT
   /* Reserve space for drive specifier and escape prefix, since either
      or both may need to be inserted.  (The Microsoft x86 compiler