Fix error when reading selection in an uneditable editor
Fix error when reading selection in an uneditable editor

(When the view isn't editable, it's possible for the anchor of the
selection to be inside it while its head is outside, which confused
the code.)

FIX: Fix issue where some DOM selections would cause a non-editable
view to crash when reading the selection.

Issue prosemirror/prosemirror#828

ProseMirror/prosemirror-view