value sets: do not try to access components of an empty struct/union

This avoids a segmentation fault while running on
c/ldv-linux-4.2-rc1/linux-4.2-rc1.tar.xz-32_7a-drivers--staging--lustre--lustre--mdc--mdc.ko-entry_point.cil.out.i
with options --pointer-check --bounds-check --unwind 2. This benchmark
includes the empty `union __anonunion_u_rpc_386`. While the patch does
address the problem on this benchmark, I failed to come up with a small
regression test.

Author: tautschnig

src/pointer-analysis/value_set.cpp

@@ -425,14 +425,17 @@ optionalt<irep_idt> value_sett::get_index_of_symbol(
     const struct_union_typet &struct_union_type =
       to_struct_union_type(followed_type);
 
-    const irep_idt &first_component_name =
-      struct_union_type.components().front().get_name();
-
-    index =
-      id2string(identifier) + "." + id2string(first_component_name) + suffix;
-    entry = find_entry(index);
-    if(entry)
-      return std::move(index);
+    if(!struct_union_type.components().empty())
+    {
+      const irep_idt &first_component_name =
+        struct_union_type.components().front().get_name();
+
+      index =
+        id2string(identifier) + "." + id2string(first_component_name) + suffix;
+      entry = find_entry(index);
+      if(entry)
+        return std::move(index);
+    }
   }
 
   // not found? try without suffix