perf subcmd: Fix missing check for return value of malloc() in add_cmdname()

perf subcmd: Fix missing check for return value of malloc() in add_cmdname()

The malloc() function may return NULL when it fails,
which may cause null pointer deference in add_cmdname(),
add Null check for return value of malloc().

Found by our static analysis tool.

Signed-off-by: Chenyuan Mi <cymi20@fudan.edu.cn>
Acked-by: Ian Rogers <irogers@google.com>
Cc: Leo Yan <leo.yan@linaro.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: linux-kernel@vger.kernel.org
Link: https://lore.kernel.org/r/20230614150118.115208-1-cymi20@fudan.edu.cn
Signed-off-by: Namhyung Kim <namhyung@kernel.org>