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 <[email protected]>
Cc: David Ahern <[email protected]>
Cc: Frederic Weisbecker <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Paul Mackerras <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Stephane Eranian <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

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