tools/python: Fix memory leak on error path

Commit 56a7aaa16b introduced a memory leak on the error path for a
Py_BuildValue built object that on some newly introduced error path
has not the correct reference count handling, fix that by decrementing
the refcount in these path.

Fixes: 56a7aaa16b ("tools: add physinfo arch_capabilities handling for Arm")
Signed-off-by: Luca Fancellu <luca.fancellu@arm.com>
Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>

tools/python/xen/lowlevel/xc/xc.c

@@ -919,11 +919,16 @@ static PyObject *pyxc_physinfo(XcObject *self)
         sve_vl_bits = arch_capabilities_arm_sve(pinfo.arch_capabilities);
         py_arm_sve_vl = PyLong_FromUnsignedLong(sve_vl_bits);
 
-        if ( !py_arm_sve_vl )
+        if ( !py_arm_sve_vl ) {
+            Py_DECREF(objret);
             return NULL;
+        }
 
-        if( PyDict_SetItemString(objret, "arm_sve_vl", py_arm_sve_vl) )
+        if( PyDict_SetItemString(objret, "arm_sve_vl", py_arm_sve_vl) ) {
+            Py_DECREF(py_arm_sve_vl);
+            Py_DECREF(objret);
             return NULL;
+        }
     }
 #endif