parse.c: quiet "may be used uninitialized" warnings/errors

gcc complains about ip_str not being initialized before the cleanup
function is called:

    In file included from /usr/include/glib-2.0/glib.h:117,
                     from ../src/parse.c:25:
    In function ‘g_autoptr_cleanup_generic_gfree’,
        inlined from ‘get_ip_family’ at ../src/parse.c:1912:22:
    /usr/include/glib-2.0/glib/glib-autocleanups.h:32:3: error: ‘ip_str’ may be used uninitialized [-Werror=maybe-uninitialized]
       32 |   g_free (*pp);
          |   ^~~~~~~~~~~~
    ../src/parse.c: In function ‘get_ip_family’:
    ../src/parse.c:1912:22: note: ‘ip_str’ was declared here
     1912 |     g_autofree char *ip_str;
          |                      ^~~~~~
    cc1: all warnings being treated as errors

Let's initialize it to NULL to satisfy the compiler.

src/parse.c

@@ -1909,7 +1909,7 @@ handle_vxlan_tristate(NetplanParser* npp, yaml_node_t* node, const void* data, G
 STATIC int
 get_ip_family(const char* address)
 {
-    g_autofree char *ip_str;
+    g_autofree char *ip_str = NULL;
     char *prefix_len;
 
     ip_str = g_strdup(address);