fdo#65163: sw: CrossRefBookmark: ensure that unused m_pPos2 is null

... to prevent ~SwIndexReg() assertion when the note it is initialized
with is deleted, since m_pPos2 will never be corrected because
CrossRefBookmark essentially denies that it exists.

Change-Id: I48543c47da0f4006e8381f4cc8a2ee81826461c9
sw: CrossRefBookmark: deny the existence of m_pPos2 a little harder

Change-Id: I32adf98474283e04b209b7ca4ec4077f908d2ecd
sw: deploy some more asserts in bookmark code

Change-Id: Ic86a81e09cf32109fe2433c3da0fedd1a96656b7
sw: s/m_rSwdoc/m_rDoc/g

... because i can type m_rDoc but "m_rSwdoc" always comes out m_rSwDoc.

Change-Id: Id50888012da6cf3cf7aebc52e8c70fc359c6e073