tags: Fixed the tag parser to correctly treat the message field as optional.

This fix makes libgit2 capable of parsing annotated tag objects that lack
the optional message/description field.
Previously, libgit2 treated this field as mandatory and raised a tag_error on
such tags. However, the message field is optional.

An example of such a tag is refs/tags/v2.6.16.31-rc1 in Linux:

$ git cat-file tag refs/tags/v2.6.16.31-rc1
object afaa018cefb6af63befef1df7d8febaae904434f
type commit
tag v2.6.16.31-rc1
tagger Adrian Bunk <bunk@stusta.de> 1162716505 +0100
$

1 files changed, 10 insertions, 7 deletions

diff --git a/src/tag.c b/src/tag.c
index 56f84a85f..4c3d811eb 100644
--- a/src/tag.c
+++ b/src/tag.c
@@ -139,16 +139,19 @@ int git_tag__parse_buffer(git_tag *tag, const char *buffer, size_t length)
 			return -1;
 	}
 
-	if( *buffer != '\n' )
-		return tag_error("No new line before message");
+	tag->message = NULL;
+	if (buffer < buffer_end) {
+		if( *buffer != '\n' )
+			return tag_error("No new line before message");
 
-	text_len = buffer_end - ++buffer;
+		text_len = buffer_end - ++buffer;
 
-	tag->message = git__malloc(text_len + 1);
-	GITERR_CHECK_ALLOC(tag->message);
+		tag->message = git__malloc(text_len + 1);
+		GITERR_CHECK_ALLOC(tag->message);
 
-	memcpy(tag->message, buffer, text_len);
-	tag->message[text_len] = '\0';
+		memcpy(tag->message, buffer, text_len);
+		tag->message[text_len] = '\0';
+	}
 
 	return 0;
 }