Don't leak memory when duplicating a NULL signature

src/blame.c

@@ -76,9 +76,7 @@ static git_blame_hunk* dup_hunk(git_blame_hunk *hunk)
 	git_oid_cpy(&newhunk->orig_commit_id, &hunk->orig_commit_id);
 	git_oid_cpy(&newhunk->final_commit_id, &hunk->final_commit_id);
 	newhunk->boundary = hunk->boundary;
-	if (hunk->final_signature)
 	newhunk->final_signature = git_signature_dup(hunk->final_signature);
-	if (hunk->orig_signature)
 	newhunk->orig_signature = git_signature_dup(hunk->orig_signature);
 	return newhunk;
 }

src/signature.c

@@ -84,8 +84,12 @@ int git_signature_new(git_signature **sig_out, const char *name, const char *ema
 
 git_signature *git_signature_dup(const git_signature *sig)
 {
-	git_signature *new = git__calloc(1, sizeof(git_signature));
+	git_signature *new;
 
+	if (sig == NULL)
+		return NULL;
+
+	new = git__calloc(1, sizeof(git_signature));
 	if (new == NULL)
 		return NULL;