perf diff: Fix error return value in __cmd_diff()

An appropriate return value should be set on the failed path.

Fixes: 2a09a84c

 ("perf diff: Support hot streams comparison")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Acked-by: Namhyung Kim <namhyung@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: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20201124103652.438-1-thunder.leizhen@huawei.com


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