Don't use -1 as the length of an integer.

That's not supported.  Put the unknown enterprise number in a a 4-byte
value, as that's what it is, and then just skip to the end of the
packet.

Change-Id: I2cce84bfbb46a114d4809dbc61a021d497eb87a8
Reviewed-on: https://code.wireshark.org/review/3708
Reviewed-by: Guy Harris <guy@alum.mit.edu>