playlist/cue/parser: fix nullptr dereference

Closes https://github.com/MusicPlayerDaemon/MPD/issues/974

NEWS

@@ -3,7 +3,7 @@ ver 0.22.1 (not yet released)
   - alsa: don't deadlock when the ALSA driver is buggy
   - jack, pulse: reduce the delay when stopping or pausing playback
 * playlist
-  - cue: fix crash bug
+  - cue: fix two crash bugs
 
 ver 0.22 (2020/09/23)
 * protocol

src/playlist/cue/CueParser.cxx

@@ -262,7 +262,9 @@ CueParser::Feed2(char *p) noexcept
 		if (previous != nullptr && previous->GetStartTime().ToMS() < (unsigned)position_ms)
 			previous->SetEndTime(SongTime::FromMS(position_ms));
 
-		current->SetStartTime(SongTime::FromMS(position_ms));
+		if (current != nullptr)
+			current->SetStartTime(SongTime::FromMS(position_ms));
+
 		if(strcmp(nr, "00") != 0 || previous == nullptr)
 			ignore_index = true;
 	}