serialize: grow strings as data arrives, not to the declared length (#18)

The vector unserializer grows in 5 MB steps so a bogus length cannot make
it allocate more than has actually been received. The string one resized
straight to whatever ReadCompactSize returned, and ReadCompactSize has no
ceiling -- nine bytes on the wire can name four gigabytes.

Not reachable from the network today: the only string on a serialized
type is CWalletTx::mapValue, and RelayMessage casts to CTransaction
before sending, so it never leaves disk. This is a guard against the next
network message that carries a string -- Bitcoin's own version message
carries strSubVer -- rather than a fix for a live hole.

0 files changed, 0 insertions, 0 deletions