QtConsole Completion Fix - Do not force the "end" to be later than the "start"

This is a follow-on fix to #6692.  By forcing `end > start`, if the buffer ended with a non-python identifier, the last character would be eaten by the result: e.g.  `import <TAB>`, select `abc`, would result in `importabc`.  With this patch, we now get `import abc`.