perf tools: No need to check if the argument to __get() function is NULL

perf tools: No need to check if the argument to __get() function is NULL

Those functions always check if the argument is NULL before trying to
grab a reference count, and also will return the received object, so, to
make code more compact, no need to check for NULL.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Krister Johansen <kjlx@templeofstupid.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: https://lkml.kernel.org/n/tip-i9wycjdxh0fwhryu55lmafks@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>