protocol/ArgParser: cast seek offset to SignedSongTime

"The issue is that ParseCommandArgSignedSongTime parses with
SongTime::FromS, not SignedSongTime::FromS, before casting back to a
SignedSongTime for the return. With x86 overflow rules this doesn't
matter, but on ARM the first cast turns negative values to zero."

Closes https://github.com/MusicPlayerDaemon/MPD/issues/757

NEWS

@@ -3,6 +3,7 @@ ver 0.21.20 (not yet released)
   - audiofile, ffmpeg, sndfile: handle MIME type "audio/wav"
   - ffmpeg: fix playback of AIFF and TTA
   - vorbis, opus: fix seeking in small files
+* fix backwards seeking on ARM (and other non-x86 CPUs)
 
 ver 0.21.19 (2020/01/17)
 * configuration

src/protocol/ArgParser.cxx

@@ -175,5 +175,5 @@ SignedSongTime
 ParseCommandArgSignedSongTime(const char *s)
 {
 	auto value = ParseCommandArgFloat(s);
-	return SongTime::FromS(value);
+	return SignedSongTime::FromS(value);
 }