test_http_server: Fix dangling pointer issue

Get(index) returns a copy of a string. That copy is kept alive by the
const reference until the end of the GetCStr function. When the GetCStr
function returns, the copy is destroyed (and so is its char buffer),
so returned pointer to the char buffer becomes dangling.

Suggested-by: Krzesimir Nowak <knowak@microsoft.com>

Author: dongsupark

src/update_engine/test_http_server.cc

@@ -466,8 +466,7 @@ class UrlTerms {
     return terms[index];
   }
   inline const char *GetCStr(const off_t index) const {
-    const string& term = Get(index);
-    return term.c_str();
+    return terms[index].c_str();
   }
   inline int GetInt(const off_t index) const {
     return atoi(GetCStr(index));