Do not attempt to perform OOB write in writeCString

Include the nul terminator in the size computation. Otherwise,
Node.js may throw an exception when `.writeUInt8` is called.

Fixes: #40

Author: addaleax

lib/ref.js

@@ -578,7 +578,7 @@ exports.writeCString = function writeCString (buffer, offset, string, encoding)
   if (!encoding) {
     encoding = 'utf8';
   }
-  const size = buffer.length - offset;
+  const size = buffer.length - offset - 1;
   const len = buffer.write(string, offset, size, encoding);
   buffer.writeUInt8(0, offset + len);  // NUL terminate
 }

test/string.js

@@ -31,6 +31,20 @@ describe('C string', function() {
       }
       assert.strictEqual(0, buf[str.length]);
     });
+
+    it('should not write the terminating 0 out of bounds', function() {
+      const wholebuf = Buffer.alloc(20, 127);
+      const buf = wholebuf.subarray(0, 10);
+      const str = 'hello world';
+      buf.writeCString(str);
+      for (let i = 0; i < buf.length - 1; i++) {
+        assert.strictEqual(str.charCodeAt(i), buf[i]);
+      }
+      assert.strictEqual(0, buf[buf.length - 1]);
+      for (let i = buf.length; i < wholebuf.length; i++) {
+        assert.strictEqual(127, wholebuf[i]);
+      }
+    });
   });
 
   describe('allocCString()', function() {