Use correct size for sizeTy[Tvector]

This bug causes ldc-developers/ldc#825 :
When using ```Type::nullAttributes``` to create a new ```TypeVector```, it will read past the end of the source ```TypeVector``` if it was allocated using ```new TypeVector``` (i.e. the actual size is used for allocation) since ```sizeof(TypeVector)``` is less than ```sizeof(TypeBasic)```.

src/mtype.c

@@ -218,6 +218,7 @@ void Type::init()
     sizeTy[Treturn] = sizeof(TypeReturn);
     sizeTy[Terror] = sizeof(TypeError);
     sizeTy[Tnull] = sizeof(TypeNull);
+    sizeTy[Tvector] = sizeof(TypeVector);
 
     initTypeMangle();