parse: fix uninitialized variable

Fixes the build on Fedora 41:

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:1918:22:
/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:1918:22: note: ‘ip_str’ was declared here
 1918 |     g_autofree char *ip_str;
      |                      ^~~~~~
cc1: all warnings being treated as errors

src/parse.c

@@ -1927,7 +1927,7 @@ 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);