feat(ssl): Do not check if client is connected if already disconnected

[me-no-dev]

This pull request makes a small fix to the NetworkClientSecure::connected() method in NetworkClientSecure.cpp to ensure the _connected flag is checked before attempting to read data.

• libraries/NetworkClientSecure/src/NetworkClientSecure.cpp: Added a conditional check for _connected in the connected() method to prevent unnecessary or invalid read operations.

Fixes: #11325

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Test Results

 76 files   76 suites   12m 49s ⏱️
 38 tests  38 ✅ 0 💤 0 ❌
241 runs  241 ✅ 0 💤 0 ❌

Results for commit 99bce2d.

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