Fix panic on a read error during the second-chance read

The read error left a nil buffer, and the deferred slice in GetChar
panicked instead of returning Fatal.

src/tui/light.go

@@ -593,10 +593,12 @@ func (r *LightRenderer) GetChar(cancellable bool) Event {
 		// follows in the buffer.
 		if ev.Type == Invalid && stillArriving(r.buffer) {
 			waited := len(r.buffer)
-			r.buffer, result, err = r.getBytes(true)
+			var buffer []byte
+			buffer, result, err = r.getBytes(true)
 			if err != nil {
 				return Event{Fatal, 0, nil}
 			}
+			r.buffer = buffer
 			if result == getCharCancelled {
 				return Event{Invalid, 0, nil}
 			}

src/tui/light_query_test.go

@@ -7,6 +7,8 @@ import (
 	"strings"
 	"testing"
 	"time"
+
+	"golang.org/x/term"
 )
 
 // Drives queryStartup against a terminal simulated by pipes, with the replies
@@ -155,3 +157,26 @@ func TestLateKeyAfterUnfinishedSequence(t *testing.T) {
 	}
 	t.Error("the late key was dropped")
 }
+
+// A read error during the second-chance read must end fzf cleanly, not panic.
+func TestSecondChanceReadError(t *testing.T) {
+	inR, inW, err := os.Pipe()
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Cleanup(func() { inR.Close() })
+	inW.WriteString("\x1b[12")
+	go func() {
+		time.Sleep(2 * defaultEscDelay * time.Millisecond)
+		inW.Close()
+	}()
+
+	out, err := os.OpenFile(os.DevNull, os.O_WRONLY, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	r := &LightRenderer{ttyin: inR, ttyout: out, escDelay: defaultEscDelay, origState: &term.State{}}
+	if ev := r.GetChar(false); ev.Type != Fatal {
+		t.Errorf("got %s, want Fatal", ev.Type.String())
+	}
+}