client: improve stripping of getline(3) newline

The previous version of this code operated under the assumption that
getline(3) lines are always \n\0 terminated. Unfortunately, this is not
the case as readline will return input which is not terminated with a
newline character if EOF is reached before encountering this newline. In
these cases, the code would falsely strip the last character. As an
example, consider the following bemenu invocation:

	printf foo | ./bemenu

This would start bemenu with `fo` instead of `foo` as a menu item. This
commit fixes this edge case and also hardens the loop body a bit by only
entering it if getline wrote more than zero characters to the buffer.

client/bemenu.c

@@ -18,10 +18,10 @@ read_items_to_menu_from_stdin(struct bm_menu *menu)
     size_t llen = 0;
     char *line = NULL;
 
-    while ((n = getline(&line, &llen, stdin)) != -1) {
-        // Remove trailing newline
-        assert(n >= 1);
-        line[n - 1] = '\0';
+    while ((n = getline(&line, &llen, stdin)) > 0) {
+        // Remove trailing newline (if any)
+        if (line[n - 1] == '\n')
+            line[n - 1] = '\0';
 
         struct bm_item *item;
         if (!(item = bm_item_new(line)))