Prevent invalid images when building Add Page dialog (BL-3819) (#1213)
Prevent invalid images when building Add Page dialog (BL-3819) (#1213)

Without this, pretty yellow dialogs pop up when you opened the
Add Pages dialog for some books.
JohnThomson/BloomDesktop,andrew-polk/BloomDesktop,StephenMcConnel/BloomDesktop,StephenMcConnel/BloomDesktop,BloomBooks/BloomDesktop,JohnThomson/BloomDesktop,andrew-polk/BloomDesktop,gmartin7/myBloomFork,gmartin7/myBloomFork,BloomBooks/BloomDesktop,gmartin7/myBloomFork,JohnThomson/BloomDesktop,StephenMcConnel/BloomDesktop,BloomBooks/BloomDesktop,StephenMcConnel/BloomDesktop,andrew-polk/BloomDesktop,JohnThomson/BloomDesktop,andrew-polk/BloomDesktop,StephenMcConnel/BloomDesktop,StephenMcConnel/BloomDesktop,BloomBooks/BloomDesktop,gmartin7/myBloomFork,gmartin7/myBloomFork,BloomBooks/BloomDesktop,BloomBooks/BloomDesktop,JohnThomson/BloomDesktop,gmartin7/myBloomFork,andrew-polk/BloomDesktop,andrew-polk/BloomDesktop