tag: avoid a double-free when parsing tags without a tagger field

The v0.99 tag in the Git repo triggers this behavior:

http://git.kernel.org/?p=git/git.git;a=tag;h=d6602ec5194c87b0fc87103ca4d67251c76f233a

Ideally, we'd allow the tag to be instantiated even though the tagger
field is missing, but this at the very least prevents libgit2 from
crashing.

To test this bug, a new repository has been added based on the test
branch in testrepo.git. It contains a "e90810b" tag that looks like
this:

    object e90810b8df3e80c413d903f631643c716887138d
    type commit
    tag e90810b

    This is a very simple tag.

src/tag.c

@@ -126,11 +126,8 @@ static int parse_tag_buffer(git_tag *tag, const char *buffer, const char *buffer
 	if (tag->tagger == NULL)
 		return GIT_ENOMEM;
 
-	if ((error = git_signature__parse(tag->tagger, &buffer, buffer_end, "tagger ", '\n')) != 0) {
-		free(tag->tag_name);
-		git_signature_free(tag->tagger);
+	if ((error = git_signature__parse(tag->tagger, &buffer, buffer_end, "tagger ", '\n')) != 0)
 		return git__rethrow(error, "Failed to parse tag");
-	}
 
 	if( *buffer != '\n' )
 		return git__throw(GIT_EOBJCORRUPTED, "Failed to parse tag. No new line before message");

tests/resources/bad_tag.git/HEAD

+ref: refs/heads/master

tests/resources/bad_tag.git/config

+[core]
+        repositoryformatversion = 0
+        filemode = true
+        bare = true
+        logallrefupdates = true

tests/resources/bad_tag.git/packed-refs

+# pack-refs with: peeled 
+eda9f45a2a98d4c17a09d681d88569fa4ea91755 refs/tags/e90810b
+^e90810b8df3e80c413d903f631643c716887138d

tests/t08-tag.c

@@ -30,6 +30,7 @@
 static const char *tag1_id = "b25fa35b38051e4ae45d4222e795f9df2e43f1d1";
 static const char *tag2_id = "7b4384978d2493e851f9cca7858815fac9b10980";
 static const char *tagged_commit = "e90810b8df3e80c413d903f631643c716887138d";
+static const char *bad_tag_id = "eda9f45a2a98d4c17a09d681d88569fa4ea91755";
 
 BEGIN_TEST(read0, "read and parse a tag from the repository")
 	git_repository *repo;
@@ -106,6 +107,22 @@ BEGIN_TEST(read2, "list all tag names from the repository matching a specified p
 	git_repository_free(repo);
 END_TEST
 
+#define BAD_TAG_REPOSITORY_FOLDER TEST_RESOURCES "/bad_tag.git/"
+
+BEGIN_TEST(read3, "read and parse a tag without a tagger field")
+	git_repository *repo;
+	git_tag *bad_tag;
+	git_oid id;
+
+	must_pass(git_repository_open(&repo, BAD_TAG_REPOSITORY_FOLDER));
+
+	git_oid_fromstr(&id, bad_tag_id);
+
+	must_fail(git_tag_lookup(&bad_tag, repo, &id));
+
+	git_repository_free(repo);
+END_TEST
+
 
 #define TAGGER_NAME "Vicent Marti"
 #define TAGGER_EMAIL "vicent@github.com"
@@ -304,6 +321,7 @@ BEGIN_SUITE(tag)
 	ADD_TEST(read0);
 	ADD_TEST(read1);
 	ADD_TEST(read2);
+	ADD_TEST(read3);
 
 	ADD_TEST(write0);
 	ADD_TEST(write2);