perf regs: Fix abort for "-I" or "--user-regs" options

Fix an issue where the `perf` tool aborts unexpectedly when running the
following command:

```
perf record -e cycles -I -- true

Usage: perf record [<options>] [<command>]
or: perf record [<options>] -- <command> [<options>]

-I, --intr-regs[=<any register>]
sample selected machine registers on interrupt, use '-I?' to list register names
```

The usage of the `-I` or `--user-regs` options without specifying any
registers should default to sampling all general-purpose registers.

However, this currently causes an abnormal termination.

The issue was introduced by commit 3d06db9bad1a ("perf regs: Refactor
use of arch__sample_reg_masks() to perf_reg_name()").

This patch resolves the problem, ensuring that the `-I` or `--user-regs`
options work as intended without causing an abort.

Fixes: 3d06db9bad1ad8e6 ("perf regs: Refactor use of arch__sample_reg_masks() to perf_reg_name()")
Reviewed-by: Ian Rogers <irogers@google.com>
Signed-off-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Alexandre Ghiti <alex@ghiti.fr>
Cc: Guo Ren <guoren@kernel.org>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Clark <james.clark@linaro.org>
Cc: John Garry <john.g.garry@oracle.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-csky@vger.kernel.org
Cc: linux-riscv@lists.infradead.org
Cc: Mike Leach <mike.leach@linaro.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Paul Walmsley <pjw@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Falcon <thomas.falcon@intel.com>
Cc: Will Deacon <will@kernel.org>
Cc: Xudong Hao <xudong.hao@intel.com>
Cc: Zide Chen <zide.chen@intel.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>