[trel] protect against invalid long TXT data keys from TREL peer (#8891)

This commit adds checks to protect against an edge-case where TXT
data entries from a TREL peer may contain invalid and long keys. If
the TXT data happens to include keys longer than `Dns::kMaxKeyLength`,
the `mKey` in `Dns::TxtEntry` will be set to `nullptr` and the entire
entry will be provided in `mValue`. This commit adds code to skip over
such entries (check that `entry.mKey` is not null) at beginning of the
loop to avoid using a potential null `mKey` in `strcmp`.

src/core/radio/trel_interface.cpp

@@ -262,6 +262,15 @@ Error Interface::ParsePeerInfoTxtData(const Peer::Info       &aInfo,
 
     while ((error = iterator.GetNextEntry(entry)) == kErrorNone)
     {
+        // If the TXT data happens to have entries with key longer
+        // than `kMaxKeyLength`, `mKey` would be `nullptr` and full
+        // entry would be placed in `mValue`. We skip over such
+        // entries.
+        if (entry.mKey == nullptr)
+        {
+            continue;
+        }
+
         if (strcmp(entry.mKey, kTxtRecordExtAddressKey) == 0)
         {
             VerifyOrExit(!parsedExtAddress, error = kErrorParse);