perf parse-events: Fix NULL check against wrong variable

perf parse-events: Fix NULL check against wrong variable

We did a null check after "tmp->symbol = strdup(...)", but we checked
"list->symbol" other than "tmp->symbol".

Reviewed-by: John Garry <john.garry@huawei.com>
Signed-off-by: Weiguo Li <liwg06@foxmail.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/tencent_DF39269807EC9425E24787E6DB632441A405@qq.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>