Fix exception nulls

[roberth]

• currentExceptionTypeName has crashed in what appears to be abi::__cxa_current_exception_type()-> ( https://github.com/hercules-ci/hercules-ci-agent/issues/417). This fixes at least the crash part. (It's still not pretty)

• Some other calls have the potential to crash similarly; also fixed.

• Add a few const so we don't have to strdup for no good reason. Should be uninteresting. (I tried - lazily - to keep working with char * but the demangle function did not like that.)

[roberth]

Ohh we were waiting for the CI configuration to be done. Shouldn't have merged yet.