st/mesa: trim calculated userbuffer size

st/mesa: trim calculated userbuffer size

In get_array_bounds we were previously defining a user buffer sized as
(nr_vertices * stride).  The trouble is that if the vertex data
occupies less than stride bytes, the extra tailing (stride - size)
bytes may extend outside the memory actually allocated by the app and
caused a segfault.

To fix this, define a the buffer bounds to be:

   ptr .. ptr + (nr-1)*stride + element_size