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>

lib/color.c

@@ -140,6 +140,9 @@ int color_fprintf(FILE *fp, enum color_attr attr, const char *fmt, ...)
 	int ret = 0;
 	va_list args;
 
+	if (fmt == NULL)
+		return 0;
+
 	va_start(args, fmt);
 
 	if (!color_is_enabled || attr == COLOR_NONE) {