Improved handling of CRYST1 records #3446 (#3469)

* Improved handling of CRYST1 records #3446

* Apply suggestions from code review
Co-authored-by: Jason Swails <jason@entos.ai>
Co-authored-by: Jason Swails <jason@entos.ai>

wrappers/python/openmm/app/internal/pdbstructure.py

@@ -196,6 +196,7 @@ class PdbStructure(object):
                 alpha = float(pdb_line[33:40])*math.pi/180.0
                 beta = float(pdb_line[40:47])*math.pi/180.0
                 gamma = float(pdb_line[47:54])*math.pi/180.0
+                if 0 not in (a_length, b_length, c_length):
                     self._periodic_box_vectors = computePeriodicBoxVectors(a_length, b_length, c_length, alpha, beta, gamma)
             elif command == "SEQRES":
                 chain_id = pdb_line[11]