Malformed bson can force a fatal error

This came up from some fuzz testing of the crate.  I don't know bson's wire protocol but from the output of bsondump it looks like the issue is it declares an illegal BSONSize and the crate still tries to allocate it and crashes.

The offending binary is: [0x06, 0xcc, 0xf9, 0x0a, 0x05, 0x00, 0x00, 0x03, 0x00, 0xff, 0xff]