two more checks, in align_sequence_to_polymer and superpose_positions

calculate_superposition() segfaulted when called with empty ResidueSpan

include/gemmi/align.hpp

@@ -35,12 +35,13 @@ inline std::vector<int> prepare_target_gapo(const ConstResidueSpan& polymer,
   return gaps;
 }
 
-// pre: !!polymer
 inline AlignmentResult align_sequence_to_polymer(
                                      const std::vector<std::string>& full_seq,
                                      const ConstResidueSpan& polymer,
                                      PolymerType polymer_type,
                                      const AlignmentScoring* scoring=nullptr) {
+  if (!polymer)
+    return AlignmentResult();
   std::map<std::string, std::uint8_t> encoding;
   if (!scoring)
     scoring = AlignmentScoring::partial_model();

include/gemmi/qcp.hpp

@@ -303,6 +303,11 @@ inline SupResult superpose_positions(const Position* pos1, const Position* pos2,
   result.center1 = qcp_calculate_center(pos1, len, weight);
   result.center2 = qcp_calculate_center(pos2, len, weight);
 
+  if (len == 0) {
+    result.rmsd = NAN;
+    return result;
+  }
+
   double wsum = 0.0;
   if (weight == nullptr)
     wsum = (double) len;