sam460ex: Correct use after free error

Commit 51b0d834 changed error handling to report file name in error
message but forgot to move freeing it after usage. Noticed by Coverity.

Fixes: CID 1394217
Reported-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NBALATON Zoltan <balaton@eik.bme.hu>
Reviewed-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: NDavid Gibson <david@gibson.dropbear.id.au>

hw/ppc/sam460ex.c

@@ -269,11 +269,12 @@ static int sam460ex_load_device_tree(hwaddr addr,
         exit(1);
     }
     fdt = load_device_tree(filename, &fdt_size);
-    g_free(filename);
     if (!fdt) {
         error_report("Couldn't load dtb file `%s'", filename);
+        g_free(filename);
         exit(1);
     }
+    g_free(filename);
 
     /* Manipulate device tree in memory. */