color: Fix ip segfault when using --color switch

color: Fix ip segfault when using --color switch

Commit d0e72011 ("ip: ipaddress.c: add support for json output")
introduced passing -1 as enum color_attr. This is not only wrong as no
color_attr has value -1, but also causes another segfault in color_fprintf()
on this setup as there is no item with index -1 in array of enum attr_colors[].
Using COLOR_CLEAR is valid option.

Reproduce with:
$ COLORFGBG='0;15' ip -c a

NOTE: COLORFGBG is environmental variable used for defining whether user
has light or dark background.
COLORFGBG="0;15" is used to ask for color set suitable for light background,
COLORFGBG="15;0" is used to ask for color set suitable for dark background.

Signed-off-by: Petr Vorel <petr.vorel@gmail.com>