device_tree: Fix potential memory leak

cppcheck error report:

device_tree.c:156: error: Memory leak: dupname

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>

device_tree.c

@@ -153,6 +153,7 @@ int qemu_devtree_add_subnode(void *fdt, const char *name)
     int retval;
 
     if (!basename) {
+        g_free(dupname);
         return -1;
     }