libs: encoder: not call ensure_num_slices inside g_assert

g_assert will take no effect when glib's G_DISABLE_ASSERT macro is
defined. The function inside the g_assert will take no effect and
we will fail to set the correct slice number.
5 jobs for no_g_assert in 10 minutes and 38 seconds
Status Job ID Name Coverage
  Preparation
passed #291923
gst indent

00:00:26

passed #291922
manifest

00:00:49

 
  Build
passed #291924
build fedora x86_64

00:06:40

passed #291925
build nodebug fedora x86_64

00:06:28

 
  Test
passed #291926
check fedora

00:03:08