use dynamic cast instead of c-style cast when casting from base type to derived class

src/SPIN/neb_spin.cpp

@@ -172,7 +172,7 @@ void NEBSpin::run()
   if (fixes.size() != 1)
     error->all(FLERR,"NEBSpin requires use of exactly one fix neb/spin instance");
 
-  fneb = (FixNEBSpin *) fixes[0];
+  fneb = dynamic_cast<FixNEBSpin *>( fixes[0]);
   if (verbose) numall =7;
   else  numall = 4;
   memory->create(all,nreplica,numall,"neb:all");