tools api: Avoid potential double free

io__getline will free the line on error but it doesn't clear the out
argument. This may lead to the line being freed twice, like in
tools/perf/util/srcline.c as detected by clang-tidy.

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/20231009183920.200859-17-irogers@google.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>

tools/lib/api/io.h

@@ -180,6 +180,7 @@ static inline ssize_t io__getline(struct io *io, char **line_out, size_t *line_l
 	return line_len;
 err_out:
 	free(line);
+	*line_out = NULL;
 	return -ENOMEM;
 }