Bugfix: prevent crash if pid not found.

I'm unsure why this has been hit yet, but this produces
proper output and prevents a segfault.

Author: ahkok

src/svg.c

@@ -1179,6 +1179,8 @@ static void svg_ps_bars(FILE *of,
                 if (ps->pid == pid)
                         break;
         }
+        if (!ps)
+                ps = ps_first;
 
         /* need to know last node first */
         ps->sample = ps->first;