Revert "qapi: Clean up superfluous null check in qapi_dealloc_type_str()"

This reverts commit 25a7017555f1b4aeb543b5d323ff4afb8f9c5437.

Turns out the argument *can* be null: QEMU now segfaults if it
receives an invalid parameter via a qmp command instead of throwing an
error.

For example:
{ "execute": "blockdev-add",
     "arguments": { "options" : { "driver": "invalid-driver" } } }

CC: qemu-stable@nongnu.org
Signed-off-by: Peter Lieven <pl@kamp.de>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

qapi/qapi-dealloc-visitor.c

@@ -131,7 +131,9 @@ static void qapi_dealloc_end_list(Visitor *v, Error **errp)
 static void qapi_dealloc_type_str(Visitor *v, char **obj, const char *name,
                                   Error **errp)
 {
-    g_free(*obj);
+    if (obj) {
+        g_free(*obj);
+    }
 }
 
 static void qapi_dealloc_type_int(Visitor *v, int64_t *obj, const char *name,