decoder/sidplay: Fix date field to have year but not company or author

Field 2 is called <released>, formerly used as <copyright>[1][2]. It is
formatted <year><space><company or author or group>, where <year> may be
<YYYY>, <YYY?>, <YY??> or <YYYY-YY>, for example "1987", "199?", "19??"
or "1985-87". The <company or author or group> may be for example Rob
Hubbard. A full field may be for example "1987 Rob Hubbard".

This change splits the <released> field at the first <space>, to retain
the <year> part.

The 51823 SID files in High Voltage SID Collection (HVSC) version 71
have the following distribution of dates:

    333 19??         11 1990-92       6 1995-99       2 2006-08
    827 198?         88 1990-93    2140 1996        530 2007
     32 1982         69 1990-94       9 1996-97      15 2007-08
      1 1982-83      49 1990-95       2 1996-98       2 2007-09
    255 1983       3467 1991          5 1996-99       1 2007-10
    677 1984         75 1991-92    1840 1997        430 2008
    775 1985         65 1991-93       4 1997-98      23 2008-09
      3 1985-86      10 1991-94    1276 1998          1 2008-12
     10 1985-87      35 1991-97       4 1998-99     631 2009
    943 1986       3320 1992        865 1999          1 2009-10
     12 1986-87      26 1992-93      24 200?        645 2010
      5 1986-89      59 1992-94     590 2000          1 2010-12
   2083 1987          1 1992-96       4 2000-01     538 2011
     31 1987-88    2996 1993        727 2001          1 2011-12
     44 1987-89      42 1993-94     875 2002        651 2012
   2510 1988         12 1993-95       2 2002-04     811 2013
    129 1988-89       2 1993-97     844 2003        790 2014
     91 1988-90    2737 1994          3 2003-05     740 2015
     58 1988-91      16 1994-95     842 2004        792 2016
   3466 1989         20 1994-96       2 2004-05     775 2017
     95 1989-90      17 1994-97     707 2005        638 2018
    150 1989-91    2271 1995          1 2005-06     284 2019
   1077 199?          2 1995-96       2 2005-07
   2834 1990          4 1995-97     785 2006
    119 1990-91       2 1995-98       6 2006-07

References:

[1] https://www.hvsc.c64.org/download/C64Music/DOCUMENTS/SID_file_format.txt
[2] https://hvsc.c64.org/info

NEWS

@@ -2,6 +2,7 @@ ver 0.21.14 (not yet released)
 * decoder
   - sidplay: show track durations in database
   - sidplay: convert tag values from Windows-1252 charset
+  - sidplay: strip text from "Date" tag
 
 ver 0.21.13 (2019/08/06)
 * input

src/decoder/plugins/SidplayDecoderPlugin.cxx

@@ -477,6 +477,30 @@ GetInfoString(const SidTuneInfo &info, unsigned i) noexcept
 	return Windows1252ToUTF8(s);
 }
 
+gcc_pure
+static AllocatedString<char>
+GetDateString(const SidTuneInfo &info) noexcept
+{
+	/*
+	 * Field 2 is called <released>, previously used as <copyright>.
+	 * It is formatted <year><space><company or author or group>,
+	 * where <year> may be <YYYY>, <YYY?>, <YY??> or <YYYY-YY>, for
+	 * example "1987", "199?", "19??" or "1985-87". The <company or
+	 * author or group> may be for example Rob Hubbard. A full field
+	 * may be for example "1987 Rob Hubbard".
+	 */
+	AllocatedString<char> release = GetInfoString(info, 2);
+
+	/* Keep the <year> part only for the date. */
+	for (size_t i = 0; release[i] != AllocatedString<char>::SENTINEL; i++)
+		if (std::isspace(release[i])) {
+			release[i] = AllocatedString<char>::SENTINEL;
+			break;
+		}
+
+	return release;
+}
+
 static void
 ScanSidTuneInfo(const SidTuneInfo &info, unsigned track, unsigned n_tracks,
 		TagHandler &handler) noexcept
@@ -498,7 +522,7 @@ ScanSidTuneInfo(const SidTuneInfo &info, unsigned track, unsigned n_tracks,
 		handler.OnTag(TAG_ARTIST, artist.c_str());
 
 	/* date */
-	const auto date = GetInfoString(info, 2);
+	const auto date = GetDateString(info);
 	if (!date.empty())
 		handler.OnTag(TAG_DATE, date.c_str());