Tolerate missing symbols in gather_needed_globals. Fixes #620

Symbols can be missing at this stage because they would be introduced during
typechecking when it notices they are undefined, which happens if the type
they were defined on is missing.

Author: smowton

src/java_bytecode/java_bytecode_language.cpp

@@ -387,9 +387,16 @@ static void gather_needed_globals(
 {
   if(e.id()==ID_symbol)
   {
-    const auto &sym=symbol_table.lookup(to_symbol_expr(e).get_identifier());
-    if(sym.is_static_lifetime)
-      needed.add(sym);
+    // If the symbol isn't in the symbol table at all, then it is defined
+    // on an opaque type (i.e. we don't have the class definition at this point)
+    // and will be created during the typecheck phase.
+    // We don't mark it as 'needed' as it doesn't exist yet to keep.
+    auto findit=symbol_table.symbols.find(to_symbol_expr(e).get_identifier());
+    if(findit!=symbol_table.symbols.end() &&
+       findit->second.is_static_lifetime)
+    {
+      needed.add(findit->second);
+    }
   }
   else
     forall_operands(opit, e)