Buffer.writeInt{L,B}E generates wrong bytes for some negative integers

[pabigot]

The common Buffer implementation for writing integer values uses a rather opaque algorithm that involves carrying a -1 adjustment into high bytes when converting negative integers. The adjustment is improperly applied when lower bytes are zero.

An example:

> var buf = new Buffer(4);
undefined
> buf.writeInt32LE(-0x10000, 0);
4
> buf.toJSON().data
[ 0, 0, 255, 255 ]
> buf.writeIntLE(-0x10000, 0, 4);
4
> buf.toJSON().data
[ 0, 255, 254, 255 ]

This bug is present in v0.12.x through HEAD.

Patch incoming.