Don't try to validate in annotateDSSP when the validator is not set (nullptr)

Fixes a segfault if one passes in a Cif datablock that didn't have `cif::pdb::reconstruct_pdbx(cif_file);` called on it.

libdssp/src/dssp-io.cpp

@@ -747,6 +747,7 @@ void annotateDSSP(cif::datablock &db, const dssp &dssp, bool writeOther, bool wr
 {
 	using namespace std::literals;
 
+	if (db.get_validator() != nullptr) {
 		auto &validator = const_cast<cif::validator &>(*db.get_validator());
 		if (validator.get_validator_for_category("dssp_struct_summary") == nullptr)
 		{
@@ -754,6 +755,7 @@ void annotateDSSP(cif::datablock &db, const dssp &dssp, bool writeOther, bool wr
 			if (dssp_extension)
 				cif::extend_dictionary(validator, *dssp_extension);
 		}
+	}
 
 	if (dssp.empty())
 	{