[joiner] fix potential message leak when Start() fails (#12394)

This commit updates `FreeJoinerFinalizeMessage()` to remove the state
check, allowing the message to be freed regardless of the current
state. It also removes the redundant cleanup in
`PrepareJoinerFinalizeMessage()` since `Joiner::Start()` handles the
cleanup upon error.

The `Joiner::Start()` method allocates the Joiner Finalize message and
then transitions the state to `kStateDiscover` before starting the
Seeker. If starting the Seeker fails, the exit label performs
cleanup, including calling `FreeJoinerFinalizeMessage()`. Previously,
`FreeJoinerFinalizeMessage()` checked that the state was `kStateIdle`
before freeing the message. Since the state had already been updated
to `kStateDiscover`, the message would not be freed, leading to a
message leak on failure.

src/core/meshcop/joiner.cpp

@@ -358,17 +358,12 @@ Error Joiner::PrepareJoinerFinalizeMessage(const char *aProvisioningUrl,
     }
 
 exit:
-    if (error != kErrorNone)
-    {
-        FreeJoinerFinalizeMessage();
-    }
-
     return error;
 }
 
 void Joiner::FreeJoinerFinalizeMessage(void)
 {
-    VerifyOrExit(mState == kStateIdle && mFinalizeMessage != nullptr);
+    VerifyOrExit(mFinalizeMessage != nullptr);
 
     mFinalizeMessage->Free();
     mFinalizeMessage = nullptr;