If we write the annotation but not the microdescriptor, rewind.

This fixes bug 9047 (and some parts of 9031, 8922, 8883 that weren't
fixed in 8822).  Bugfix on 0.2.2.6-alpha.

src/common/compat.h

@@ -411,6 +411,7 @@ tor_lockfile_t *tor_lockfile_lock(const char *filename, int blocking,
 void tor_lockfile_unlock(tor_lockfile_t *lockfile);
 
 off_t tor_fd_getpos(int fd);
+int tor_fd_setpos(int fd, off_t pos);
 int tor_fd_seekend(int fd);
 
 #ifdef _WIN32