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.

sed.c

@@ -451,7 +451,7 @@ read_line(FILE *f, String *s)
 
 	if ((len = getline(&s->str, &s->cap, f)) < 0) {
 		if (ferror(f))
-			eprintf("getline failed\n");
+			eprintf("getline:");
 		return EOF;
 	}
 	if (s->str[--len] == '\n')