perf machine: Fix the value used for unknown pids

perf machine: Fix the value used for unknown pids

The value used for unknown pids cannot be zero because that is used by
the "idle" task.

Use -1 instead.  Also handle the unknown pid case when creating map
groups.

Note that, threads with an unknown pid should not occur because fork (or
synthesized) events precede the thread's existence.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/1405332185-4050-2-git-send-email-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>