Fix an uninitialized-read when parsing v3 introduction requests.

Fortunately, later checks mean that uninitialized data can't get sent
to the network by this bug.  Unfortunately, reading uninitialized heap
*can* (in some cases, with some allocators) cause a crash if you get
unlucky and go off the end of a page.

Found by asn.  Bugfix on 0.2.4.1-alpha.