perf annotate: Fix heap overflow

perf annotate: Fix heap overflow

Fix expand_tabs that copies the source lines '\0' and then appends
another '\0' at a potentially out of bounds address.

Signed-off-by: Ian Rogers <irogers@google.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jin Yao <yao.jin@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Song Liu <songliubraving@fb.com>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lore.kernel.org/lkml/20191026035644.217548-1-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>