rgw: Initialize pointer field in ReadParams

Fixes the Coverity Scan Report:
CID 1353428: Uninitialized pointer field (UNINIT_CTOR)
2. uninit_member: Non-static class member cache_info is not initialized in this constructor nor in any functions that it calls.

Signed-off-by: Jos Collin <jcollin@redhat.com>

src/rgw/rgw_rados.h

@@ -2669,7 +2669,7 @@ class RGWRados
       } stat_params;
 
       struct ReadParams {
-        rgw_cache_entry_info *cache_info;
+        rgw_cache_entry_info *cache_info{nullptr};
         map<string, bufferlist> *attrs;
 
         ReadParams() : attrs(NULL) {}