draw: fix building error in draw_gs_init()

draw: fix building error in draw_gs_init()

Fixes the following building error happening with Android build system:

external/mesa/src/gallium/auxiliary/draw/draw_gs.c:740:79:
error: address of array 'draw->gs.tgsi.machine->PrimitiveOffsets' will always evaluate to 'true' [-Werror,-Wpointer-bool-conversion]
         if (!draw->gs.tgsi.machine->Primitives[i] || !draw->gs.tgsi.machine->PrimitiveOffsets)
                                                      ~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~
1 error generated.

Fixes: 7720ce3 ("draw: add support to tgsi paths for geometry streams. (v2)")
Signed-off-by: Mauro Rossi <issor.oruam@gmail.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>