If value is defined and not "U", try to ensure it looks like a number

This is a "quick fix" not to pass non-numeric data to "RRD".

Log line changes from:

    2014-07-07 19:05:08 [error]: In RRD: Error updating /var/lib/munin/localdomain/localhost.localdomain-df_inode-_dev_sda2-g.rrd: /var/lib/munin/localdomain/localhost.localdomain-df_inode-_dev_sda2-g.rrd: conversion of '-' to float not complete: tail '-'

to

    2014-07-08 19:00:10 [warning]: Expected number, got "-": group=localdomain host=localhost.localdomain plugin=df_inode field=_dev_sda2


However, a "more correct fix" may be to introduce better validation in munin-update, so we do not end up with these kind of values in the first place.  May be more work, though.