Prefix variable nMaxFieldIndex with m_ to show it's a data member of the
class.

Change-Id: I207b3552f460b3433af6892abb6c4693e990d7bb
Reviewed-on: https://gerrit.libreoffice.org/68887
Tested-by: Jenkins
Reviewed-by: Jens Carl <j.carl43@gmx.de>