Fix exception handling for PDT operator.

Assets/Cryville/Crtr/PdtEvaluator.cs

@@ -363,6 +363,8 @@ namespace Cryville.Crtr {
 		}
 		unsafe static class oputil {
 			public static float AsNumber(PropSrc src) {
+				if (src == null)
+					throw new ArgumentNullException("src");
 				int type; byte[] value;
 				src.Get(out type, out value);
 				if (type != PdtInternalType.Number)