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.

comm.c

@@ -62,7 +62,7 @@ main(int argc, char *argv[])
 		for (i = 0; i < 2; i++) {
 			if (diff && i == (diff < 0))
 				continue;
-			if (getline(&line[i], &linelen[i], fp[i]) >= 0)
+			if (getline(&line[i], &linelen[i], fp[i]) > 0)
 				continue;
 			if (ferror(fp[i]))
 				eprintf("getline %s:", argv[i]);