perf session: Fix infinite loop on invalid perf.data file

perf-record updates the header in the perf.data file at termination.
Without this update perf-report (and other processing built-ins) it
caused an infinite loop when perf report (or something like) called.

This is because the algorithm in __perf_session__process_events()
depends on the data_size which is read from file header.  Use file size
directly instead in this case to do the best-effort processing.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Tested-by: David Ahern <dsahern@gmail.com>
Tested-by: Sonny Rao <sonnyrao@chromium.org>
Acked-by: Ingo Molnar <mingo@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Sonny Rao <sonnyrao@chromium.org>
Link: http://lkml.kernel.org/r/1380529188-27193-1-git-send-email-namhyung@kernel.org
Signed-off-by: David Ahern <dsahern@gmail.com>
[ Reworded warning as per Ingo Molnar suggestion, replaces 'perf.data'
  with session->filename, to precisely identify the data file involved ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
diff --git a/tools/perf/util/session.c b/tools/perf/util/session.c
index 37c4718..568b750 100644
--- a/tools/perf/util/session.c
+++ b/tools/perf/util/session.c
@@ -1312,7 +1312,7 @@
 	file_offset = page_offset;
 	head = data_offset - page_offset;
 
-	if (data_offset + data_size < file_size)
+	if (data_size && (data_offset + data_size < file_size))
 		file_size = data_offset + data_size;
 
 	progress_next = file_size / 16;