Hide the fact we're not printing two levels of trace

Refs trac:538

This commit was SVN r12885.

The following Trac tickets were found above:
  Ticket 538 --> https://svn.open-mpi.org/trac/ompi/ticket/538

opal/util/stacktrace.c

@@ -329,7 +329,7 @@ static void opal_show_stackframe (int signo, siginfo_t * info, void * p)
         for (i = 2 ; i < traces_size ; ++i) {
             ret = snprintf(print_buffer, sizeof(print_buffer),
                            HOSTFORMAT "[%2d] %s\n",
-                           hostname, getpid(), i, traces[i]);
+                           hostname, getpid(), i - 2, traces[i]);
             write(fileno(stderr), print_buffer, ret);
         }
     } else {