QCache: Fix crash observed in tst_QAccessibility

Fixes a use-after-free which can reliably be observed under ASAN. In
QConfFileSettingsPrivate::~QConfFileSettingsPrivate we call
unusedCache->insert(conf_file->name, conf_file, ...)
Note that the key is a member of the object. Thus by deleting the object
before using the key, we dereference a dangling pointer.

Amends f08492c6fd9818c7d80b1725355453e179b4d85b.

Pick-to: dev 6.0.0
Change-Id: I3a550fc73446b72dd46456232e85f6d206d64c01
Reviewed-by: Andy Shaw <andy.shaw@qt.io>

src/corelib/tools/qcache.h

@@ -237,8 +237,8 @@ public:
     bool insert(const Key &key, T *object, qsizetype cost = 1)
     {
         if (cost > mx) {
-            delete object;
             remove(key);
+            delete object;
             return false;
         }
         trim(mx - cost);