fix: guard against invalid handles

GetPrev() or GetNext() may return invalid residue handles. It is
unclear what happens next, find() may simply (hopefully) find nothing,
however this feels unsafe. This fix guards against this possibility
by checking for handles validity before giving them to find().

modules/mol/base/src/transfer_connectivity.cc

@@ -110,24 +110,28 @@ public:
     }
     // educated guess: we are trying to connect to the previous/next residue
     std::map<ResidueHandle, ResidueHandle>::const_iterator j;
-    j = to_from_->find(dst_res.GetPrev());
-    if (j != to_from_->end()) {
-      if (j->second == r) {
-        if (CheckInsertionCode(
-                              j->first.FindAtom(src_atom.GetName()).GetResidue(),
-                              r, dst_res)) {
-          return j->first.FindAtom(src_atom.GetName());
+    if (dst_res.GetPrev().IsValid()) {
+      j = to_from_->find(dst_res.GetPrev());
+      if (j != to_from_->end()) {
+        if (j->second == r) {
+          if (CheckInsertionCode(
+                                j->first.FindAtom(src_atom.GetName()).GetResidue(),
+                                r, dst_res)) {
+            return j->first.FindAtom(src_atom.GetName());
+          }
         }
       }
     }
-    j = to_from_->find(dst_res.GetNext());
-    if (j != to_from_->end()) {
-      if (j->second == r) {
-        if (CheckInsertionCode(
-                              j->first.FindAtom(src_atom.GetName()).GetResidue(),
-                              r, dst_res)) {
-            return j->first.FindAtom(src_atom.GetName());
-          }
+    if (dst_res.GetNext().IsValid()) {
+      j = to_from_->find(dst_res.GetNext());
+      if (j != to_from_->end()) {
+        if (j->second == r) {
+          if (CheckInsertionCode(
+                                j->first.FindAtom(src_atom.GetName()).GetResidue(),
+                                r, dst_res)) {
+              return j->first.FindAtom(src_atom.GetName());
+            }
+        }
       }
     }
     // still nothing. scan linearly through all residues.