udev: fix invalid free() in enable_name_policy()

static bool enable_name_policy(...) in ./src/udev/net/link-config.c
calls proc_cmdline(...) to get "line" initialized, but
proc_cmdline(...) does not guarantee that atleast when both
conditions (detect_container(NULL) > 0) and
read_full_file(...) returned < 0.

src/udev/net/link-config.c

@@ -185,7 +185,7 @@ static int load_link(link_config_ctx *ctx, const char *filename) {
 }
 
 static bool enable_name_policy(void) {
-        _cleanup_free_ char *line;
+        _cleanup_free_ char *line = NULL;
         char *w, *state;
         int r;
         size_t l;