perf test pmu: Set uninitialized PMU alias to null

perf test pmu: Set uninitialized PMU alias to null

Commit 3e0bf9fde2984469 ("perf pmu: Restore full PMU name wildcard
support") adds a test case "PMU cmdline match" that covers PMU name
wildcard support provided by function perf_pmu__match(). The test works
with a wide range of supported combinations of PMU name matching but
omits the case that if the perf_pmu__match() cannot match the PMU name
to the wildcard, it tries to match its alias. However, this variable is
not set up, causing the test case to fail when run with subprocesses or
to segfault if run as a single process.

  ./perf test -vv 9
    9: Sysfs PMU tests                                                 :
    9.1: Parsing with PMU format directory                             : Ok
    9.2: Parsing with PMU event                                        : Ok
    9.3: PMU event names                                               : Ok
    9.4: PMU name combining                                            : Ok
    9.5: PMU name comparison                                           : Ok
    9.6: PMU cmdline match                                             : FAILED!

  ./perf test -F 9
    9.1: Parsing with PMU format directory                             : Ok
    9.2: Parsing with PMU event                                        : Ok
    9.3: PMU event names                                               : Ok
    9.4: PMU name combining                                            : Ok
    9.5: PMU name comparison                                           : Ok
  Segmentation fault (core dumped)

Initialize the PMU alias to null for all tests of perf_pmu__match()
as this functionality is not being tested and the alias matching works
exactly the same as the matching of the PMU name.

  ./perf test -F 9
    9.1: Parsing with PMU format directory                             : Ok
    9.2: Parsing with PMU event                                        : Ok
    9.3: PMU event names                                               : Ok
    9.4: PMU name combining                                            : Ok
    9.5: PMU name comparison                                           : Ok
    9.6: PMU cmdline match                                             : Ok

Fixes: 3e0bf9fde2984469 ("perf pmu: Restore full PMU name wildcard support")
Signed-off-by: Veronika Molnarova <vmolnaro@redhat.com>
Cc: james.clark@arm.com
Cc: mpetlan@redhat.com
Cc: rstoyano@redhat.com
Link: https://lore.kernel.org/r/20240808103749.9356-1-vmolnaro@redhat.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>