Correctly pass a concatenation elaboration error.

Because Icarus tries to elaborate as much as it can even after
an error has occurred we need to check for these errors during
elaboration. This patch prevent an undefined identifier from
crashing the compiler.

elaborate.cc

@@ -2649,8 +2649,8 @@ NetProc* PEventStatement::elaborate_st(Design*des, NetScope*scope,
 	    }
 	    NexusSet*nset = enet->nex_input(rem_out);
 	    if (nset == 0) {
-		  cerr << get_fileline() << ": internal error: No NexusSet"
-		       << " from statement." << endl;
+		  cerr << get_fileline() << ": error: Unable to elaborate:"
+		       << endl;
 		  enet->dump(cerr, 6);
 		  des->errors += 1;
 		  return enet;

net_nex_input.cc

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2002-2007 Stephen Williams (steve@icarus.com)
+ * Copyright (c) 2002-2008 Stephen Williams (steve@icarus.com)
  *
  *    This source code is free software; you can redistribute it
  *    and/or modify it in source code form under the terms of the GNU
@@ -53,8 +53,13 @@ NexusSet* NetEBinary::nex_input(bool rem_out)
 
 NexusSet* NetEConcat::nex_input(bool rem_out)
 {
+      if (parms_[0] == NULL) return NULL;
       NexusSet*result = parms_[0]->nex_input(rem_out);
       for (unsigned idx = 1 ;  idx < parms_.count() ;  idx += 1) {
+	    if (parms_[idx] == NULL) {
+		  delete result;
+		  return NULL;
+	    }
 	    NexusSet*tmp = parms_[idx]->nex_input(rem_out);
 	    result->add(*tmp);
 	    delete tmp;
@@ -98,6 +103,10 @@ NexusSet* NetESelect::nex_input(bool rem_out)
 {
       NexusSet*result = base_? base_->nex_input(rem_out) : new NexusSet();
       NexusSet*tmp = expr_->nex_input(rem_out);
+      if (tmp == NULL) {
+	    delete result;
+	    return NULL;
+      }
       result->add(*tmp);
       delete tmp;
       return result;
@@ -381,4 +390,3 @@ NexusSet* NetWhile::nex_input(bool rem_out)
       delete tmp;
       return result;
 }
-