[joiner] keep discovering when UDP port is bad (#11337)

This commit fixes the issue that the joiner starts joining process
immediately when an invalid Joiner UDP Port is encountered. This issue
was introduced by #9445 which attempts to ignore joiner routers having
an invalid Joiner UDP Port.

src/core/meshcop/joiner.cpp

@@ -249,7 +249,7 @@ void Joiner::HandleDiscoverResult(Mle::DiscoverScanner::ScanResult *aResult)
 {
     VerifyOrExit(mState == kStateDiscover);
 
-    if (aResult != nullptr && aResult->mJoinerUdpPort > 0)
+    if (aResult != nullptr)
     {
         SaveDiscoveredJoinerRouter(*aResult);
     }
@@ -270,9 +270,11 @@ void Joiner::SaveDiscoveredJoinerRouter(const Mle::DiscoverScanner::ScanResult &
 {
     uint8_t       priority;
     bool          doesAllowAny;
-    JoinerRouter *end = GetArrayEnd(mJoinerRouters);
+    JoinerRouter *end;
     JoinerRouter *entry;
 
+    VerifyOrExit(aResult.mJoinerUdpPort > 0);
+
     doesAllowAny = AsCoreType(&aResult.mSteeringData).PermitsAllJoiners();
 
     LogInfo("Joiner discover network: %s, pan:0x%04x, port:%d, chan:%d, rssi:%d, allow-any:%s",
@@ -284,6 +286,8 @@ void Joiner::SaveDiscoveredJoinerRouter(const Mle::DiscoverScanner::ScanResult &
     // We keep the list sorted based on priority. Find the place to
     // add the new result.
 
+    end = GetArrayEnd(mJoinerRouters);
+
     for (entry = &mJoinerRouters[0]; entry < end; entry++)
     {
         if (priority > entry->mPriority)