mraajs.i: add buffer checks to python write() typemaps in Spi & I2c

mraajs.i: add buffer checks to python write() typemaps in Spi & I2c

Previously anything passed to an I2c::write() or Spi::write() function in
node.js that wasn't a node::Buffer or an object would likely cause a segfault
later on during future calls. This change calls node::Buffer::HasInstance(obj)
to make sure the buffer is valid prior to grabbing it's data

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>