perf trace: Mark syscall ids that are not allocated to avoid unnecessary error messages

perf trace: Mark syscall ids that are not allocated to avoid unnecessary error messages

There are holes in syscall tables with IDs not associated with any
syscall, mark those when trying to read information for syscalls, which
could happen when iterating thru all syscalls from 0 to the highest
numbered syscall id.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Luis Cláudio Gonçalves <lclaudio@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-cku9mpcrcsqaiq0jepu86r68@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>