Fix isc_nm_httpconnect to check for shuttindown condition

Merged Ondřej Surý requested to merge ondrej/fix-isc_nm_httpconnect-to-check-for-shuttingdown into main

The isc_nm_httpconnect() would succeed even if the netmgr would be already shuttingdown. This has been fixed and the unit test has been updated to cope with fact that the handle would be NULL when isc_nm_httpconnect() returns with an error.

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