libgcrypt/mpi: Fix possible NULL dereference

The code in gcry_mpi_scan() assumes that buffer is not NULL, but there
is no explicit check for that, so we add one.

Fixes: CID 73757

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

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

@@ -379,6 +379,9 @@ gcry_mpi_scan (struct gcry_mpi **ret_mpi, enum gcry_mpi_format format,
   unsigned int len;
   int secure = (buffer && gcry_is_secure (buffer));
 
+  if (!buffer)
+    return gcry_error (GPG_ERR_INV_ARG);
+
   if (format == GCRYMPI_FMT_SSH)
     len = 0;
   else