perf machine: Avoid out of bounds LBR memory read

Running perf top with address sanitizer and "--call-graph=lbr" fails
due to reading sample 0 when no samples exist. Add a guard to prevent
this.

Fixes: e2b23483eb1d ("perf machine: Factor out lbr_callchain_add_lbr_ip()")
Signed-off-by: Ian Rogers <[email protected]>
Cc: K Prateek Nayak <[email protected]>
Cc: Ravi Bangoria <[email protected]>
Cc: Sandipan Das <[email protected]>
Cc: Anshuman Khandual <[email protected]>
Cc: German Gomez <[email protected]>
Cc: James Clark <[email protected]>
Cc: Nick Terrell <[email protected]>
Cc: Sean Christopherson <[email protected]>
Cc: Changbin Du <[email protected]>
Cc: liuwenyu <[email protected]>
Cc: Yang Jihong <[email protected]>
Cc: Masami Hiramatsu <[email protected]>
Cc: Miguel Ojeda <[email protected]>
Cc: Song Liu <[email protected]>
Cc: Leo Yan <[email protected]>
Cc: Kajol Jain <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Kan Liang <[email protected]>
Cc: Athira Rajeev <[email protected]>
Cc: Yanteng Si <[email protected]>
Cc: Liam Howlett <[email protected]>
Cc: Paolo Bonzini <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Namhyung Kim <[email protected]>

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