perf record: Fix memory leak when using '--user-regs=?' to list registers

When using 'perf record's option '-I' or '--user-regs=' along with
argument '?' to list available register names, memory of variable 'os'
allocated by strdup() needs to be released before __parse_regs()
returns, otherwise memory leak will occur.

Fixes: bcc84ec6

 ("perf record: Add ability to name registers to record")
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Li Bin <huawei.libin@huawei.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20200703093344.189450-1-zhengzengkai@huawei.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>