DynamicTablesPkg: Fix SMBIOS Type 7 string table cleanup

Check the return status from StringTableInitialize() before using the
SMBIOS Type 7 string table.

Free the string table when adding the socket designation string fails and
after publishing the final string set into the SMBIOS record.

Also free any Type 7 records already added to TableList on the common
error path before freeing TableList itself.

Signed-off-by: Varshit Pandya <varshit.pandya@arm.com>