Avoid assignment inside the macro call

This causes multiple assignments, which has undefined sequences

Author: zsx

src/core/c-do.c

@@ -449,7 +449,8 @@ void Trace_Arg(REBINT num, REBVAL *arg, REBVAL *path)
 	pvs.store = DS_TOP;		// Temp space for constructed results
 
 	// Get first block value:
-	pvs.path = VAL_BLK_DATA(pvs.orig = *path_val);
+	pvs.orig = *path_val;
+	pvs.path = VAL_BLK_DATA(pvs.orig);
 
 	// Lookup the value of the variable:
 	if (IS_WORD(pvs.path)) {