Fix exception handling for PDT operator.

Assets/Cryville/Common/Pdt/PdtOperator.cs

@@ -47,12 +47,12 @@ namespace Cryville.Common.Pdt {
 		internal void Call(byte* prmem, bool noset) {
 			_prmem = prmem;
 			_rfreq = false;
-			Execute();
-			if (!_rfreq && !noset) throw new InvalidOperationException("Return frame not set");
+			try { Execute(); } catch (Exception) { _failure = true; }
 			if (_failure) {
 				if (_rfreq) _etor.DiscardStack();
 				throw new InvalidOperationException("Evaluation failed");
 			}
+			if (!_rfreq && !noset) throw new InvalidOperationException("Return frame not set");
 		}
 		/// <summary>
 		/// Executes the 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)