Check first, as machines__deliver_event() may have
perf_evlist__id2evsel() returning NULL.

This was found while checking a report from Leo Yan that used the smatch
tool to find places where a pointer is checked before use and then,
later in the same function gets used without checking.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Leo Yan <leo.yan@linaro.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-muvb8xqyh0gysgfjfq35w642@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>