perf sort: Fix segfault with basic block 'cycles' sort dimension

perf sort: Fix segfault with basic block 'cycles' sort dimension

Skip the sample which doesn't have branch_info to avoid segmentation
fault:

The fault can be reproduced by:

  perf record -a
  perf report -F cycles

Signed-off-by: Changbin Du <changbin.du@intel.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Fixes: 0e332f033a82 ("perf tools: Add support for cycles, weight branch_info field")
Link: http://lkml.kernel.org/r/20170313083148.23568-1-changbin.du@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>