perf metric: Return early if no CPU PMU table exists

Previous behavior is to segfault if there is no CPU PMU table and a
metric is sought. To reproduce compile with NO_JEVENTS=1 then request a
metric, for example, "perf stat -M IPC true".

Committer testing:

Before:

  $ make -k NO_JEVENTS=1 BUILD_BPF_SKEL=1 O=/tmp/build/perf-urgent -C tools/perf install-bin
  $ perf stat -M IPC true
  Segmentation fault (core dumped)
  $

After:

  $ perf stat -M IPC true

   Usage: perf stat [<options>] [<command>]

      -M, --metrics <metric/metric group list>
                            monitor specified metrics or metric groups (separated by ,)
  $

Fixes: 00facc760903be66 ("perf jevents: Switch build to use jevents.py")
Signed-off-by: Ian Rogers <[email protected]>
Tested-by: Arnaldo Carvalho de Melo <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Caleb Biggers <[email protected]>
Cc: Florian Fischer <[email protected]>
Cc: Ian Rogers <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: James Clark <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: John Garry <[email protected]>
Cc: Kan Liang <[email protected]>
Cc: Kshipra Bopardikar <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Miaoqian Lin <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Perry Taylor <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Stephane Eranian <[email protected]>
Cc: Thomas Richter <[email protected]>
Cc: Xing Zhengjun <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

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