Don't read broken data after 'head' pointer.

Following commits will feed perf_evlist__mmap_read() with some 'head'
pointers not maintained by kernel. If 'head' pointer breaks an event, we
should avoid reading from the broken event. This can happen in backward
ring buffer.

For example:

                              old     head
                                |     |
                                V     V
     +---+------+----------+----+-----+--+
     |..E|D....D|C........C|B..B|A....|E.|
     +---+------+----------+----+-----+--+

'old' pointer points to the beginning of 'A' and trying read from it,
but 'A' has been overwritten. In this case, don't try to read from 'A',
simply return NULL.
Signed-off-by: Wang Nan <wangnan0@huawei.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Zefan Li <lizefan@huawei.com>
Cc: pi3orama@163.com
Link: http://lkml.kernel.org/r/1461637738-62722-2-git-send-email-wangnan0@huawei.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>