fix leak when getline fails

according to the getline(3) documentation, the calling code needs to
free the buffer even if getline fails.

dmenu currently doesn't do that which results in a small leak in case of
failure (e.g when piped /dev/null)

	$ ./dmenu < /dev/null
	==8201==ERROR: LeakSanitizer: detected memory leaks
	Direct leak of 120 byte(s) in 1 object(s) allocated from:
	    #0 0x7f6bf5785ef7 in malloc
	    #1 0x7f6bf538ec84 in __getdelim
	    #2 0x405d0c in readstdin dmenu.c:557

moving `line = NULL` inside the loop body wasn't strictly necessary, but
IMO it makes it more apparent that `line` is getting cleared to NULL
after each successful iteration.

dmenu.c

@@ -554,7 +554,7 @@ readstdin(void)
 	ssize_t len;
 
 	/* read each line from stdin and add it to the item list */
-	for (i = 0; (len = getline(&line, &junk, stdin)) != -1; i++, line = NULL) {
+	for (i = 0; (len = getline(&line, &junk, stdin)) != -1; i++) {
 		if (i + 1 >= size / sizeof *items)
 			if (!(items = realloc(items, (size += BUFSIZ))))
 				die("cannot realloc %zu bytes:", size);
@@ -562,7 +562,9 @@ readstdin(void)
 			line[len - 1] = '\0';
 		items[i].text = line;
 		items[i].out = 0;
+		line = NULL;
 	}
+	free(line);
 	if (items)
 		items[i].text = NULL;
 	lines = MIN(lines, i);