perf top/report: Fix infinite loop in the TUI for grouped events

For a while we need to have a dummy event for doing things like
receiving PERF_RECORD_COMM, PERF_RECORD_EXEC, etc for threads being
created and dying while we synthesize the pre-existing ones at tool
start.

This 'dummy' event is needed for keeping track of thread lifetime events
early in the session but are uninteresting otherwise, i.e. no need to
have it in a initial events menu for the non-grouped case, i.e. for:

 # perf top -e cycles,instructions

or even for plain:

 # perf top

When 'cycles' and that 'dummy' event are in place.

The code to remove that 'dummy' event ended up creating an endless loop
for the grouped case, i.e.:

 # perf top -e '{cycles,instructions}'

Fix it.

Fixes: bee9ca1c ("perf report TUI: Remove needless 'dummy' event from menu")
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>