perf tools: Fix out-of-bound access to struct perf_session

perf tools: Fix out-of-bound access to struct perf_session

If filename is NULL there is an out-of-bound access to struct
perf_session if it would be used with perf_session__open(). Shouldn't
actually happen in current implementation as filename is always
!NULL. Fixing this by always null-terminating filename.

Signed-off-by: Robert Richter <robert.richter@amd.com>