handle post policy only if it is set. (#6852)

Previously policy in post form is assumed to be set always.  This is
fixed by doing the check when policy is set.

cmd/bucket-handlers.go

@@ -572,6 +572,8 @@ func (api objectAPIHandlers) PostPolicyBucketHandler(w http.ResponseWriter, r *h
 		return
 	}
 
+	// Handle policy if it is set.
+	if len(policyBytes) > 0 {
 		postPolicyForm, err := parsePostPolicyForm(string(policyBytes))
 		if err != nil {
 			writeErrorResponse(w, ErrMalformedPOSTRequest, r.URL, guessIsBrowserReq(r))
@@ -598,6 +600,7 @@ func (api objectAPIHandlers) PostPolicyBucketHandler(w http.ResponseWriter, r *h
 				return
 			}
 		}
+	}
 
 	// Extract metadata to be saved from received Form.
 	metadata := make(map[string]string)