public final void writeVarLengthSigned(final int v) {
        writeVarLengthUnsigned(v < 0 ? ((-v) << 1) | 1 : v << 1);
    }
Just reserves a single byte and return it' offset.
Used in conjunction with injectVarLengthUnsigned
to efficiently write a size prefix

@return the offset of the placeholder
main/src/main/java/cgeo/geocaching/brouter/util/ByteDataWriter.java