stumpless_get_severity_enum_from_buffer broken for incomplete names

As described in #535, the implementation of `stumpless_get_severity_enum_from_buffer` is broken, returning enumeration values for incomplete types when `strncasecmp` is properly implemented. This needs to be addressed both to fix behavior, and also determine why the custom `strncasecmp` implementation is not already causing the `GetSeverityEnumFromBuffer.IncompleteSeverity` test to fail.