[9.20] fix: usr: Importing invalid SKR file might corrupt stack memory

If an BIND 9 administrator imports an invalid SKR file, local stack
in the import function might overflow.  This could lead to
a memory corruption on the stack and ultimately server crash.
This has been fixed.

ISC would like to thank mcsky23 for bringing this bug to our attention.

Closes #5758

Backport of MR !11578

Merge branch 'backport-5758-fix-stack-overflow-via-rndc-skr-import-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11598