Fix isConstructor to avoid exception

Author: liufengyun

compiler/src/dotty/tools/dotc/core/classfile/ClassfileParser.scala

@@ -245,7 +245,7 @@ class ClassfileParser(
       val sym = denot.symbol
       val jflags = in.nextChar
       val isEnum = (jflags & JAVA_ACC_ENUM) != 0
-      val name = pool.getName(in.nextChar)
+      val name = pool.getName(in.nextChar).name
       val isConstructor = name eq nme.CONSTRUCTOR
 
       /** Strip leading outer param from constructor and trailing access tag for