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>

src/javascript/mraajs.i

@@ -11,11 +11,17 @@
 %}
 
 %typemap(in) (const uint8_t *data, int length) {
+  if (!node::Buffer::HasInstance($input)) {
+      SWIG_exception_fail(SWIG_ERROR, "Expected a node Buffer");
+  }
   $1 = (uint8_t*) node::Buffer::Data($input);
   $2 = node::Buffer::Length($input);
 }
 
 %typemap(in) (uint8_t *txBuf, int length) {
+  if (!node::Buffer::HasInstance($input)) {
+      SWIG_exception_fail(SWIG_ERROR, "Expected a node Buffer");
+  }
   $1 = (uint8_t*) node::Buffer::Data($input);
   $2 = node::Buffer::Length($input);
 }