procstat_getpathname: for kvm method, instead of returning the error

that the method is not supported, return an empty string.

This looks more handy for callers like procstat(1), which will not
abort after the failed call and still output some useful information.

MFC after:	3 weeks

lib/libprocstat/libprocstat.c

@@ -2114,8 +2114,10 @@ procstat_getpathname(struct procstat *procstat, struct kinfo_proc *kp,
 {
 	switch(procstat->type) {
 	case PROCSTAT_KVM:
-		warnx("kvm method is not supported");
-		return (-1);
+		/* XXX: Return empty string. */
+		if (maxlen > 0)
+			pathname[0] = '\0';
+		return (0);
 	case PROCSTAT_SYSCTL:
 		return (procstat_getpathname_sysctl(kp->ki_pid, pathname,
 		    maxlen));