Bug 19258: Prevent warn when paying a fine or charge

Bug 19258: Prevent warn when paying a fine or charge

To test:
 1) Go to a members detail page in staff client
 2) Select the Fines tab in the left pane
 3) Select the Create manual invoice tab below the button menu bar
 4) Create a fine and click save (e.g. Type: Fine, Amount: 5.00)
 5) Select the Pay fines tab below the button menu bar
 6) Click Pay on the item
 7) Blank the staff error log
 8) click confirm
    -- staff error log has message
 9) apply this first patch
10) repeat steps 3-8
    -- staff error log is blank
11) run koha qa test tools

Sponsored-by: Catalyst IT
Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>