perf parse-events: Fix incorrect conversion of 'if () free()' to 'zfree()'

perf parse-events: Fix incorrect conversion of 'if () free()' to 'zfree()'

When applying a patch by Ian I incorrectly converted to zfree() an
expression that involved testing some other struct member, not the one
being freed, which lead to bugs reproduceable by:

  $ perf stat -e i/bs,tsc,L2/o sleep 1
  WARNING: multiple event parsing errors
  Segmentation fault (core dumped)
  $

Fix it by restoring the test for pos->free_str before freeing
pos->val.str, but continue using zfree(&pos->val.str) to set that member
to NULL after freeing it.

Reported-by: Ian Rogers <irogers@google.com>
Fixes: e8dfb81838b1 ("perf parse-events: Fix memory leaks found on parse_events")
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: clang-built-linux@googlegroups.com
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Leo Yan <leo.yan@linaro.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>