Check getline()-return-values properly

It's not useful when 0 is returned anyway, so be sure that we have a
string with length > 0, this also solves some indexing-gotchas like
"len - 1" and so on.
Also, add checked getline()'s whenever it has been forgotten and
clean up the error-messages.

logger.c

@@ -70,7 +70,7 @@ main(int argc, char *argv[])
 	openlog(tag ? tag : getlogin(), logflags, 0);
 
 	if (!argc) {
-		while (getline(&buf, &sz, stdin) >= 0)
+		while (getline(&buf, &sz, stdin) > 0)
 			syslog(priority, "%s", buf);
 		if (ferror(stdin))
 			eprintf("getline %s:", "<stdin>");