strace-graph: cope with clone immediately followed by exit

* strace-graph: A clone() followed by exit() with no intervening fork
or execve resulted in the new pid having no seq key.  That breaks when
trying to use seq as an array reference.  Ensure that clone populates
the new pid with an empty seq entry.
diff --git a/strace-graph b/strace-graph
index ae355aa..680eb5f 100755
--- a/strace-graph
+++ b/strace-graph
@@ -212,7 +212,7 @@
 # process info, indexed by pid.
 # fields:
 #    parent         pid number
-#    seq            forks and execs for this pid, in sequence  (array)
+#    seq            clones, forks and execs for this pid, in sequence  (array)
 
 #  filename and argv (from latest exec)
 #  basename (derived from filename)
@@ -250,6 +250,7 @@
 	push @$seq, ['FORK', $result];
 	$pr{$pid}{seq} = $seq;
 	$pr{$result}{parent} = $pid;
+	$pr{$result}{seq} = [];
     } elsif ($call eq '_exit' || $call eq 'exit_group') {
 	$pr{$pid}{end} = $time if defined $time;
     }