softpipe: check if so_target is NULL before accessing it

softpipe: check if so_target is NULL before accessing it

No idea if this is working right but copied straight from llvmpipe.
(Not only does this check the so_target but also use buffer->data instead
of buffer for the mapping.)
Just trying to get rid of a segfault testing something else...

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Reviewed-by: Zack Rusin <zackr@vmware.com>