[EagerAppCDS] fix bug by clearing buffer before using

Summary: as title

Testing: /test/com/alibaba/quickstart/TestEnvVariableReplay.java

Reviewers: lingjun-cg, yulei

Issue: dragonwell-project#646

hotspot/src/share/vm/runtime/quickStart.cpp

@@ -1097,6 +1097,7 @@ void QuickStart::convert_path_by_env(const char* origin_path, char* new_path) {
   }
 
   int p1 = 0, p2 = p1;
+  new_path[0] = '\0';
   bool process_end = false;
   while (!process_end) {
     if (origin_path[p2] == '\0') {