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: NDavid S. Miller <davem@davemloft.net>
Link: http://lkml.kernel.org/r/20120325.162822.2267799792062571623.davem@davemloft.netSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/symbol.c

@@ -50,6 +50,8 @@ struct symbol_conf symbol_conf = {
 
 int dso__name_len(const struct dso *dso)
 {
+	if (!dso)
+		return strlen("[unknown]");
 	if (verbose)
 		return dso->long_name_len;