Fix a bug in Dbg::GetClassDescriptor.

The callers were incorrectly assuming we always returned a descriptor, which
isn't true when handed bad input.

Change-Id: I5509d18d8c234d97fa5ec2e0a01d4cdb44acfc55
diff --git a/src/jdwp/jdwp_main.cc b/src/jdwp/jdwp_main.cc
index e408a76..a7e82aa 100644
--- a/src/jdwp/jdwp_main.cc
+++ b/src/jdwp/jdwp_main.cc
@@ -452,7 +452,7 @@
 
 std::ostream& operator<<(std::ostream& os, const JdwpLocation& rhs) {
   os << "JdwpLocation["
-     << Dbg::GetClassDescriptor(rhs.classId) << "." << Dbg::GetMethodName(rhs.classId, rhs.methodId)
+     << Dbg::GetClassName(rhs.classId) << "." << Dbg::GetMethodName(rhs.classId, rhs.methodId)
      << "@" << rhs.idx << " " << rhs.typeTag << "]";
   return os;
 }