perf trace: Avoid an ERR_PTR in syscall_stats

perf trace: Avoid an ERR_PTR in syscall_stats

hashmap__new may return an ERR_PTR and previously this would be
assigned to syscall_stats meaning all use of syscall_stats needs to
test for NULL (uninitialized) or an ERR_PTR. Given the only reason
hashmap__new can fail is ENOMEM, just use NULL to indicate the
allocation failure and avoid the code having to test for NULL and
IS_ERR.

Fixes: 96f202eab813 (perf trace: Fix IS_ERR() vs NULL check bug)
Signed-off-by: Ian Rogers <irogers@google.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>