[BUGFIX] indexed_search/Indexer: correct frequency mapping for 1

There was a slight glitch in indexed_search's indexer, which resulted in
a wrong frequency mapping if the ratio between number of current word /
all words is exactly 1.
The "frequency" for this word is now mapped to the highest possible
value.

Resolves: #47745
Releases: master, 6.2
Change-Id: I1c9945b9550b80ad969f907f0bf1f14c3ae872f9
Reviewed-on: http://review.typo3.org/32993
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>