perf symbols: Handle NULL dso in dso__name_len

We should use "[unknown]" in this case, in concert with the code in
_hist_entry__dso_snprintf().

Otherwise we'll crash when recomputing the histogram column lengths in
hists__calc_col_len().

Signed-off-by: David S. Miller <davem@davemloft.net>
Link: http://lkml.kernel.org/r/20120325.162822.2267799792062571623.davem@davemloft.net


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>