Fix some really dumb segfaults if link endpoints didn't exist.

assign/parse_ptop.cc

@@ -193,6 +193,16 @@ int parse_ptop(tb_pgraph &PG, tb_sgraph &SG, istream& i)
       }
 
 #define ISSWITCH(n) (n->types.find("switch") != n->types.end())
+      // Check to make sure the nodes in the link actually exist
+      if (pname2vertex.find(src) == pname2vertex.end()) {
+	  ptop_error("Bad link line, non-existent node.");
+	  continue;
+      }
+      if (pname2vertex.find(dst) == pname2vertex.end()) {
+	  ptop_error("Bad link line, non-existent node.");
+	  continue;
+      }
+
       pvertex srcv = pname2vertex[src];
       pvertex dstv = pname2vertex[dst];
       tb_pnode *srcnode = get(pvertex_pmap,srcv);

assign/parse_top.cc

@@ -162,6 +162,16 @@ int parse_top(tb_vgraph &VG, istream& i)
 	}
 
 	vedge e;
+	// Check to make sure the nodes in the link actually exist
+	if (vname2vertex.find(src) == vname2vertex.end()) {
+	  top_error("Bad link line, non-existent node.");
+	  continue;
+	}
+	if (vname2vertex.find(dst) == vname2vertex.end()) {
+	  top_error("Bad link line, non-existent node.");
+	  continue;
+	}
+
 	vvertex node1 = vname2vertex[src];
 	vvertex node2 = vname2vertex[dst];
 	e = add_edge(node1,node2,VG).first;