Update iedit-lib.el
clean up old occurrences when calling from isearch

If iedit-mode-from-isearch is called when iedit is already active, 
it's possible to have overlays for two different patterns at the same time.
This cleans up the old occurrences before making new ones.
Merge pull request #32 from mbneedham/master

Small bugfix to clean up old occurrences when calling from isearch