color: handle case where fmt is NULL

color: handle case where fmt is NULL

There are cases where NULL is passed as format string when
nothing is to be printed. This is commonly done in the print_bool
function when a flag is false. Glibc seems to handle this case nicely
but for musl it will cause a segmentation fault

Since nothing needs to be printed, in this case; just check
for NULL and return.

Reported-by: Pedro Tammela <pctammela@mojatatu.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>