Fixed use after free in ModSecurity::processContentOffset

- Use after free issue detected with Address Sanitizer while running
  the reading_logs_with_offset example.
- Keeps reference to last element in vars vector with vars.back(). Then
  it removes the element from vars calling vars.pop_back() which
  invalidates the reference, but it's accessed later in the function.
  - Fixed by copying the value instead.

src/modsecurity.cc

@@ -262,9 +262,9 @@ int ModSecurity::processContentOffset(const char *content, size_t len,
         std::string value;
         yajl_gen_map_open(g);
         vars.pop_back();
-        const std::string &startingAt = vars.back().str();
+        const std::string startingAt = vars.back().str();
         vars.pop_back();
-        const std::string &size = vars.back().str();
+        const std::string size = vars.back().str();
         vars.pop_back();
         yajl_gen_string(g,
             reinterpret_cast<const unsigned char*>("startingAt"),
@@ -346,9 +346,9 @@ int ModSecurity::processContentOffset(const char *content, size_t len,
             strlen("highlight"));
         yajl_gen_map_open(g);
         ops.pop_back();
-        std::string startingAt = ops.back().str();
+        const std::string startingAt = ops.back().str();
         ops.pop_back();
-        std::string size = ops.back().str();
+        const std::string size = ops.back().str();
         ops.pop_back();
         yajl_gen_string(g,
             reinterpret_cast<const unsigned char*>("startingAt"),