Skip FZF_CURRENT_ITEM export when item contains NUL

- exec(2) rejects env entries containing NUL, breaking preview and
  other child commands when the input has NUL bytes
- skip the export and document the limitation

Fix #2395

man/man1/fzf.1

@@ -1531,6 +1531,10 @@ fzf exports the following environment variables to its child processes.
 .br
 .BR FZF_RAW "             Only in raw mode. 1 if the current item matches, 0 otherwise"
 
+.PP
+.B FZF_CURRENT_ITEM
+is omitted when the item contains a NUL byte, because exec(2) cannot pass it.
+
 .SH EXTENDED SEARCH MODE
 
 Unless specified otherwise, fzf will start in "extended\-search mode". In this

src/terminal.go

@@ -1439,7 +1439,10 @@ func (t *Terminal) environImpl(forPreview bool) []string {
 	env = append(env, fmt.Sprintf("FZF_COLUMNS=%d", t.areaColumns))
 	env = append(env, fmt.Sprintf("FZF_POS=%d", min(t.merger.Length(), t.cy+1)))
 	if item := t.currentItem(); item != nil {
-		env = append(env, "FZF_CURRENT_ITEM="+item.AsString(t.ansi))
+		// Skip if the value contains a NUL byte; exec(2) would reject the env.
+		if s := item.AsString(t.ansi); !strings.ContainsRune(s, 0) {
+			env = append(env, "FZF_CURRENT_ITEM="+s)
+		}
 	}
 	env = append(env, fmt.Sprintf("FZF_CLICK_HEADER_LINE=%d", t.clickHeaderLine))
 	env = append(env, fmt.Sprintf("FZF_CLICK_HEADER_COLUMN=%d", t.clickHeaderColumn))