[mesh-forwarder] update CheckReachablity() to handle malformed messages (#9976)

This commit ensures `MeshForwarder::CheckReachablity()` rejects frames
where `Ip6::Header::DecompressFrom(FrameData)` returns a parse error.
This prevents the processing of potentially malformed messages.

src/core/thread/mesh_forwarder_ftd.cpp

@@ -627,10 +627,16 @@ Error MeshForwarder::CheckReachability(const FrameData &aFrameData, const Mac::A
 
     error = ip6Headers.DecompressFrom(aFrameData, aMeshAddrs, GetInstance());
 
-    if (error == kErrorNotFound)
+    switch (error)
     {
+    case kErrorNone:
+        break;
+    case kErrorNotFound:
         // Frame may not contain an IPv6 header.
-        ExitNow(error = kErrorNone);
+        error = kErrorNone;
+        OT_FALL_THROUGH;
+    default:
+        ExitNow();
     }
 
     error = Get<Mle::MleRouter>().CheckReachability(aMeshAddrs.mDestination.GetShort(), ip6Headers.GetIp6Header());