fs.write crashes on non-integer fd

[monsanto]

fs.write(null, new Buffer(1), 0, 1)

iojs: ../src/node_file.cc:782: void node::WriteBuffer(const v8::FunctionCallbackInfo<v8::Value>&): Assertion `args[0]->IsInt32()' failed.
Aborted (core dumped)

[cjihrig]

Fixed in f9c681c