idmapd: Fix decoding of octal encoded fields

The decoded octal will always be positive and (char) -1 is negative. Any
field containing an encoded octal will be rejected.

As the encoded value should be an unsigned char, fix the check to reject
all values > (unsigned char) -1 = UCHAR_MAX, as this indicate an error
in the encoding.

Signed-off-by: Jan-Marek Glogowski <glogow@fbihome.de>
Signed-off-by: Steve Dickson <steved@redhat.com>