Fix a bug in resolver's resume_dslookup() function

Arаm Sаrgsyаn requested to merge 3839-fix-resume_dslookup-bug into main

A recent refactoring in 7e4e125e had introduced a logical error which could result in calling the dns_resolver_createfetch() function with 'nameservers' pointer set to NULL, but with 'domain' not set to NULL, which is not allowed by the function.

Make sure 'domain' is set only when 'nsrdataset' is valid.

Closes #3839 (closed)

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