bug#14729: 24.3.50; Isearch oddity

[Juri Linkov]

> I expect that the fontification of the echo area reflects what is
> happening: that the whole search string ("buffer") is unmatched (not
> only the last "fer"), since the current search begins just after the
> last instance of "buf" in the buffer [1], and there is no match
> (neither full nor partial) of the current search string between that
> location and the end of the buffer.

`isearch-fail-pos' assumes that in a failed state of search
a shorter previous successful search string should be taken
as a basis for highlighting.  In this case, the last successful match
was "buf", so an appended suffix is highlighted as failed.