Fix failure of non-incremental document saving.
Fix failure of non-incremental document saving.

An earlier commit changed the mode used to open a file for saving so that
it could also be read from. The mode used was rb+ independently of whether
the saving mode was incremental or not. Doing so neglected that for
non-incemental saves the file may not already exist in which case opening
rb+ will fail. This commit arranges that wb+ is used in the non-incremental
case.

ccxvii/mupdf,sebras/mupdf,muennich/mupdf,fluks/mupdf-x11-bookmarks,ArtifexSoftware/mupdf,TamirEvan/mupdf,fluks/mupdf-x11-bookmarks,ccxvii/mupdf,muennich/mupdf,fluks/mupdf-x11-bookmarks,TamirEvan/mupdf,ArtifexSoftware/mupdf,fluks/mupdf-x11-bookmarks,muennich/mupdf,TamirEvan/mupdf,TamirEvan/mupdf,ccxvii/mupdf,fluks/mupdf-x11-bookmarks,TamirEvan/mupdf,ArtifexSoftware/mupdf,fluks/mupdf-x11-bookmarks,sebras/mupdf,ccxvii/mupdf,TamirEvan/mupdf,sebras/mupdf,sebras/mupdf,ArtifexSoftware/mupdf,fluks/mupdf-x11-bookmarks,TamirEvan/mupdf,ArtifexSoftware/mupdf,muennich/mupdf,TamirEvan/mupdf,ccxvii/mupdf,sebras/mupdf,ArtifexSoftware/mupdf,ArtifexSoftware/mupdf,ArtifexSoftware/mupdf,muennich/mupdf,muennich/mupdf,muennich/mupdf,ccxvii/mupdf,sebras/mupdf