perf auxtrace: Fix potential NULL pointer dereference

In the function auxtrace_parse_snapshot_options(), the callback pointer
"itr->parse_snapshot_options" can be NULL if it has not been set during
the AUX record initialization.  This can cause tool crashing if the
callback pointer "itr->parse_snapshot_options" is dereferenced without
performing NULL check.

Add a NULL check for the pointer "itr->parse_snapshot_options" before
invoke the callback.

Fixes: d20031bb ("perf tools: Add AUX area tracing Snapshot Mode")
Signed-off-by: Leo Yan <leo.yan@linaro.org>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Tiezhu Yang <yangtiezhu@loongson.cn>
Link: http://lore.kernel.org/lkml/20210420151554.2031768-1-leo.yan@linaro.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>