perf tools: Avoid warning in do_realloc_array_as_needed()

do_realloc_array_as_needed() used memcpy() of zero size with a NULL
pointer. Check the size first to avoid sanitize warning.

Discovered using EXTRA_CFLAGS="-fsanitize=undefined -fsanitize=address".

Reported-by: kernel test robot <yujie.liu@intel.com>
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Acked-by: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/oe-lkp/202303061424.6ad43294-yujie.liu@intel.com
Link: https://lore.kernel.org/r/20230316194156.8320-2-adrian.hunter@intel.com


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