qapi: fix error propagation

Don't overwrite / leak previously set errors.
Make traversal cope with missing mandatory sub-structs.
Don't try to end a container that could not be started.

v1->v2:
- unchanged

v2->v3:
- instead of examining, assert that we never overwrite errors with
  error_set()
- allow visitors to set a NULL struct pointer successfully, so traversal
  of incomplete objects can continue
- check for a NULL "obj" before accessing "(*obj)->has_XXX" (this is not a
  typo, "obj != NULL" implies "*obj != NULL" here)
- fix start_struct / end_struct balance for unions as well

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>

error.c

@@ -32,6 +32,7 @@ void error_set(Error **errp, const char *fmt, ...)
     if (errp == NULL) {
         return;
     }
+    assert(*errp == NULL);
 
     err = g_malloc0(sizeof(*err));
 
@@ -132,7 +133,7 @@ bool error_is_type(Error *err, const char *fmt)
 
 void error_propagate(Error **dst_err, Error *local_err)
 {
-    if (dst_err) {
+    if (dst_err && !*dst_err) {
         *dst_err = local_err;
     } else if (local_err) {
         error_free(local_err);