perf pmu: Avoid uninitialized use of alias->str

perf pmu: Avoid uninitialized use of alias->str

alias is allocated with malloc allowing uninitialized memory to be
accessed.

The initialization of str was moved late after it could have been
updated by a JSON event, however, this create a potential for an
uninitialized use.

Fix this by assigning str to NULL early.

Testing on ARM (Raspberry Pi) showed a memory leak in the same code so
add a zfree.

Fixes: f63a536f03a2f64f ("perf pmu: Merge JSON events with sysfs at load time")
Reported-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Ian Rogers <irogers@google.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Clark <james.clark@arm.com>
Cc: Jing Zhang <renyu.zj@linux.alibaba.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sumanth Korikkar <sumanthk@linux.ibm.com>
Cc: Thomas Richter <tmricht@linux.ibm.com>
Link: https://lore.kernel.org/r/20230830000545.1638964-1-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>