mesa: Update _Current ptr along Current when a new vertex program is bind.

mesa: Update _Current ptr along Current when a new vertex program is bind.

On new vertex program bind only the Current ptr where updated to
point to the new program; this could lead to _Current pointing to
and out of date or even a no more existing program which in turn
could lead to a segfault in some driver. To avoid this _Current
ptr is updated along Current if _Current where previously pointing
to the same program.