st/mesa: Validate the result of pipe_transfer_map in make_texture (v2)

When using Freecad, I was getting intermittent segfaults inside of
mesa.  I traced it down to this path in st_cb_drawpixels.c where the
result of pipe_transfer_map wasn't being checked.  In my case, it was
returning NULL because nouveau_bo_new returned ENOENT.  I'm by no
means a mesa developer, but this patch solves the problem for me and
seems reasonable enough.

v2: Marek - also unmap the PBO and release the texture, and call
    the make_texture function sooner for less cleanup

Cc: 18.1 18.2 <mesa-stable@lists.freedesktop.org>
(cherry picked from commit 936e0dcd)