libgcrypt/mpi: Fix possible unintended sign extension

The array of unsigned char gets promoted to a signed 32-bit int before
it is finally promoted to a size_t. There is the possibility that this
may result in the signed-bit being set for the intermediate signed
32-bit int. We should ensure that the promotion is to the correct type
before we bitwise-OR the values.

Fixes: CID 96697

Signed-off-by: Darren Kenny <darren.kenny@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/lib/libgcrypt/mpi/mpicoder.c

@@ -458,7 +458,7 @@ gcry_mpi_scan (struct gcry_mpi **ret_mpi, enum gcry_mpi_format format,
       if (len && len < 4)
         return gcry_error (GPG_ERR_TOO_SHORT);
 
-      n = (s[0] << 24 | s[1] << 16 | s[2] << 8 | s[3]);
+      n = ((size_t)s[0] << 24 | (size_t)s[1] << 16 | (size_t)s[2] << 8 | (size_t)s[3]);
       s += 4;
       if (len)
         len -= 4;