Fix crash when attempting to replace a placeholder with NULL

For example, this solves the combination of an explicit format string
'@a@ - @t@' in combination with an MP3 input file that lacks either
artist or title

NEWS

+Changes in 1.0.2, released on XXXX-XX-XX:
+
+ * Fix a crash, if metadata placeholders are configured for input files
+   that do not contain the respective values. From gui-lux on Github (#16).
+
+
+
 Changes in 1.0.1, released on 2020-02-22:
 
  * Fix a crash, if there is a typo in the <encoder /> setting

src/mdata.c

@@ -483,15 +483,15 @@ mdata_strformat(struct mdata *md, char *buf, size_t bufsize, const char *format)
 
 	memset(dicts, 0, sizeof(dicts));
 	dicts[0].from = PLACEHOLDER_ARTIST;
-	dicts[0].to = mdata_get_artist(md);
+	dicts[0].to = mdata_get_artist(md) ? mdata_get_artist(md) : "";
 	dicts[1].from = PLACEHOLDER_ALBUM;
-	dicts[1].to = mdata_get_album(md);
+	dicts[1].to = mdata_get_album(md) ? mdata_get_album(md) : "";
 	dicts[2].from = PLACEHOLDER_TITLE;
-	dicts[2].to = mdata_get_title(md);
+	dicts[2].to = mdata_get_title(md) ? mdata_get_title(md) : "";
 	dicts[3].from = PLACEHOLDER_TRACK;
 	dicts[3].to = mdata_get_filename(md);
 	dicts[4].from = PLACEHOLDER_STRING;
-	dicts[4].to = mdata_get_songinfo(md);
+	dicts[4].to = mdata_get_songinfo(md) ? mdata_get_songinfo(md) : "";
 
 	str = util_expand_words(format, dicts);
 	ret = (int)strlen(str);

tests/check_mdata.c

@@ -147,15 +147,38 @@ START_TEST(test_mdata_strformat)
 	int	ret;
 
 	ck_assert_int_eq(mdata_parse_file(md, SRCDIR "/test01-artist+album+title.ogg"), 0);
-
-	ck_assert_int_lt(mdata_strformat(md, buf, sizeof(buf), NULL), 0);
-
 	ret = mdata_strformat(md, buf, sizeof(buf), "@a@/@b@/@t@/@T@/@s@");
 	ck_assert_int_eq(ret, (int)strlen(buf));
 	ck_assert_str_eq(buf,
 	    "test artist/test album/test title"
 	    "/" SRCDIR "/test01-artist+album+title.ogg"
 	    "/test artist - test title - test album");
+
+	ck_assert_int_eq(mdata_parse_file(md, SRCDIR "/test12-artist.ogg"), 0);
+	ret = mdata_strformat(md, buf, sizeof(buf), "@a@/@b@/@t@/@T@/@s@");
+	ck_assert_int_eq(ret, (int)strlen(buf));
+	ck_assert_str_eq(buf,
+	    "test artist//"
+	    "/" SRCDIR "/test12-artist.ogg"
+	    "/test artist");
+
+	ck_assert_int_eq(mdata_parse_file(md, SRCDIR "/test15-title.ogg"), 0);
+	ret = mdata_strformat(md, buf, sizeof(buf), "@a@/@b@/@t@/@T@/@s@");
+	ck_assert_int_eq(ret, (int)strlen(buf));
+	ck_assert_str_eq(buf,
+	    "//test title"
+	    "/" SRCDIR "/test15-title.ogg"
+	    "/test title");
+
+	ck_assert_int_eq(mdata_parse_file(md, SRCDIR "/test16-nometa.ogg"), 0);
+	ret = mdata_strformat(md, buf, sizeof(buf), "@a@/@b@/@t@/@T@/@s@");
+	ck_assert_int_eq(ret, (int)strlen(buf));
+	ck_assert_str_eq(buf,
+	    "//"
+	    "/" SRCDIR "/test16-nometa.ogg"
+	    "/");
+
+	ck_assert_int_lt(mdata_strformat(md, buf, sizeof(buf), NULL), 0);
 }
 END_TEST