kernel: fix crash when tracing buggy attributes/properties

If an attribute or property returns nothing (resp. something other than true
or false), this is an error, which we normally catch. But when tracing is
enabled for the relevant operations, we would instead crash.

src/opers.c

@@ -2561,7 +2561,7 @@ Obj DoAttribute (
     val = DoOperation1Args( self, obj );
     while (val == (Obj) 0) {
         val = ErrorReturnObj("Method for an attribute must return a value",
-                             0L, 0L, 
+                             0L, 0L,
                              "you can supply a value <val> via 'return <val>;'");
     }
     val = CopyObj( val, 0 );
@@ -2614,6 +2614,11 @@ Obj DoVerboseAttribute (
 
     /* call the operation to compute the value                             */
     val = DoVerboseOperation1Args( self, obj );
+    while (val == (Obj) 0) {
+        val = ErrorReturnObj("Method for an attribute must return a value",
+                             0L, 0L,
+                             "you can supply a value <val> via 'return <val>;'");
+    }
     val = CopyObj( val, 0 );
 
     /* set the value (but not for internal objects)                        */
@@ -2983,12 +2988,12 @@ Obj DoProperty (
     /* call the operation to compute the value                             */
     val = DoOperation1Args( self, obj );
     while ( val != True && val != False ) {
-        val = ErrorReturnObj( 
+        val = ErrorReturnObj(
                "Method for a property did not return true or false",
-               0L, 0L, 
+               0L, 0L,
                "you can 'return true;' or 'return false;'");
     }
-    
+
     /* set the value (but not for internal objects)                        */
     if ( ENABLED_ATTR(self) == 1 && ! IS_MUTABLE_OBJ(obj) ) {
         switch ( TNUM_OBJ( obj ) ) {
@@ -3038,7 +3043,13 @@ Obj DoVerboseProperty (
 
     /* call the operation to compute the value                             */
     val = DoVerboseOperation1Args( self, obj );
-
+    while ( val != True && val != False ) {
+        val = ErrorReturnObj(
+               "Method for a property did not return true or false",
+               0L, 0L,
+               "you can 'return true;' or 'return false;'");
+    }
+
     /* set the value (but not for internal objects)                        */
     if ( ENABLED_ATTR(self) == 1 && ! IS_MUTABLE_OBJ(obj) ) {
         switch ( TNUM_OBJ( obj ) ) {