gallium/util: Make u_prims_for_vertices() safe

gallium/util: Make u_prims_for_vertices() safe

Let us avoid trapping in hardware from a SIGFPE and instead
assert on a zero divisor.

Hint: This can occur if a PIPE_PRIM_? is not handled in
      u_prim_vertex_count() that results in ' info ' not
      being initialized in the expected manner.

Further, we also fix a possibly NULL pointer dereference
from ' info ' being NULL from a u_prim_vertex_count() call.

Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Signed-off-by: Marek Olšák <marek.olsak@amd.com>