Fixed struct initialization error on older versions of C#

"'this' object cannot be used before all of its fields are assigned to"

Change-Id: Icccdcc0d0be0fe0b87abe0eb28fe1cc91116fcfb

net/FlatBuffers/Struct.cs

@@ -25,7 +25,7 @@ namespace FlatBuffers
         public ByteBuffer bb { get; private set; }
 
         // Re-init the internal state with an external buffer {@code ByteBuffer} and an offset within.
-        public Struct(int _i, ByteBuffer _bb)
+        public Struct(int _i, ByteBuffer _bb) : this()
         {
             bb = _bb;
             bb_pos = _i;

net/FlatBuffers/Table.cs

@@ -31,7 +31,7 @@ namespace FlatBuffers
         public ByteBuffer ByteBuffer { get { return bb; } }
 
         // Re-init the internal state with an external buffer {@code ByteBuffer} and an offset within.
-        public Table(int _i, ByteBuffer _bb)
+        public Table(int _i, ByteBuffer _bb) : this()
         {
             bb = _bb;
             bb_pos = _i;