perf bench uprobe: Fix potential use of memory after free

Found by clang-tidy:
```
bench/uprobe.c:98:3: warning: Use of memory after it is freed [clang-analyzer-unix.Malloc]
                bench_uprobe_bpf__destroy(skel);
```
Signed-off-by: Ian Rogers <irogers@google.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Ravi Bangoria <ravi.bangoria@amd.com>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Yang Jihong <yangjihong1@huawei.com>
Cc: Huacai Chen <chenhuacai@kernel.org>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: llvm@lists.linux.dev
Cc: Ming Wang <wangming01@loongson.cn>
Cc: Tom Rix <trix@redhat.com>
Cc: bpf@vger.kernel.org
Link: https://lore.kernel.org/r/20231009183920.200859-6-irogers@google.comSigned-off-by: Namhyung Kim <namhyung@kernel.org>

tools/perf/bench/uprobe.c

@@ -89,6 +89,7 @@ static int bench_uprobe__setup_bpf_skel(enum bench_uprobe bench)
 	return err;
 cleanup:
 	bench_uprobe_bpf__destroy(skel);
+	skel = NULL;
 	return err;
 }