Updated the documents to give warning to the users of `findHomography` that the function may return an empty matrix in some cases. 
The user must take care of checking that.

Add the ensureSimpleDistance() method to ensure the user the returned distance is not ^2 (the default for L2 for instance)

This fixes all problems from the article "Checking OpenCV with PVS-Studio"
<http://www.viva64.com/en/b/0191/> that are not already fixed and are
not in 3rdparty or the legacy module.

The problems fixed are two instances of useless code and one instance
of unspecified behavior (right-shifting a negative number).