tests: Use error, not assert, when trying to print a non-base type DIE.

tests: Use error, not assert, when trying to print a non-base type DIE.

When using the varlocs test with a fuzzer using assert for internal
sanity checks is great to find issues. But when encountering bad data
using an assert is wrong. Just use error to show we handle the data
correctly (by reporting it is bad, instead of crashing).

Signed-off-by: Mark Wielaard <mark@klomp.org>