Merge pull request #4306 from libgit2/cmn/tag-bad-signature

signature: don't leave a dangling pointer to the strings on parse failure

src/signature.c

@@ -231,6 +231,7 @@ int git_signature__parse(git_signature *sig, const char **buffer_out,
 		if (git__strtol64(&sig->when.time, time_start, &time_end, 10) < 0) {
 			git__free(sig->name);
 			git__free(sig->email);
+			sig->name = sig->email = NULL;
 			return signature_error("invalid Unix timestamp");
 		}