Handle Push element error

[robUx4]

Not sure if we should make the EbmlMaster constructor throw in this case. There might be some code assuming Mandatory elements are available no matter what. This is also needed in the copy constructor of #162. But it would break the API/ABI so not for 1.x.

This only happens when pushing to a vector fails, so extremely limited memory constraints.

[mbunkus]

Two things:

Indentation — can you please use two spaces, even for the first new indentation level you introduced?

Nesting — can you please convert the code not to use all of those nesting layers? It's easy to either continue; or return or whatever earlier by inverting the tests. E.g. (untested!)

  for (EltIdx = 0; EltIdx < EBML_CTX_SIZE(MasterContext); EltIdx++) {
    if (!EBML_CTX_IDX(MasterContext,EltIdx).IsMandatory() && EBML_CTX_IDX(MasterContext,EltIdx).IsUnique())
      continue;

//  assert(EBML_CTX_IDX(MasterContext,EltIdx).Create != NULL);

    if (PushElement(EBML_SEM_CREATE(EBML_CTX_IDX(MasterContext,EltIdx))))
      continue;

    while (EltIdx-- != 0) {
      if (!EBML_CTX_IDX(MasterContext,EltIdx).IsMandatory() && EBML_CTX_IDX(MasterContext,EltIdx).IsUnique())
        continue;

      // element that have been pushed should be removed
      auto *todelete = ElementList.back();
      ElementList.pop_back();
      delete todelete;
    }

    return false;
  }

[robUx4]

Fixed the Indentation and Nesting.