Rename multiple files: Determine correctly if the name pattern is valid

The "Rename" button in the dialog should be enabled if and only if the
"new name" pattern is valid. This is the case if the pattern contains
exactly one sequence of '#', which will be replaced by digits.

This patch fixes the problem that
(a) A pattern that contains a single '#' is not considered valid, and
(b) A pattern without any '#' at all is not recognized as invalid.

BUG: 318942
FIXED-IN: 4.10.3
REVIEW: 110223