R12 emulator crashes with zero-length port_control binary

[Simon Cornish]

If a linked-in driver returns 0 to a port_control call and
PORT_CONTROL_FLAG_BINARY is set then the beam emulator will probably
crash or otherwise misbehave.

Attached is a patch for those who are stuck on R12 and might get
bitten by this. Tested on R12B-3, applies also to R12B-5.

It's already fixed (in a different way) in R13+

/Simon
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