Assure miniscript holds only T expression

Author: joemphilips

NBitcoin/Miniscript/Miniscript.cs

@@ -48,6 +48,8 @@ public Miniscript(AstElem astElem)
 		{
 			if (astElem == null)
 				throw new System.ArgumentNullException(nameof(astElem));
+			if (!astElem.IsT())
+				throw new ArgumentException($"AstElem is not TopLevel expression! AstElem: {astElem}");
 			Policy = astElem.ToPolicy();
 			ast = astElem;
 		}

NBitcoin/Miniscript/MiniscriptScriptParser.cs

@@ -385,6 +385,6 @@ private static P PostProcess(AstElem ast)
 			PAstElemCore.Bind(ast => Parse.Ref(() => PostProcess(ast)));
 
 		public static AstElem ParseScript(Script sc)
-			=> PAstElem.Parse(sc);
+			=> PT.Parse(sc);
 	}
 }