st/mesa: check for dummy programs in destroy_program_variants()

st/mesa: check for dummy programs in destroy_program_variants()

When we destroy an ARB vp/fp whose ID was gen'd but not otherwise used we
get a pointer to the dummy/placeholder program.  We can't destroy that one
so just skip it.  This only failed during context tear-down because
glDeleteProgramsARB() was already aware of dummy programs.

Fixes https://bugs.freedesktop.org/show_bug.cgi?id=38086

Note: This is a candidate for the stable branches.

Tested-by: Andreas Boll <andreas.boll.dev@gmail.com>