Do not call assign_from_json if not in symbol_table

The assign_from_json will fail with an invariant failure if it is called
on a method name that does not exist in the symbol table so we should be
careful when calling it.

Author: romainbrenguier

jbmc/src/java_bytecode/java_static_initializers.cpp

@@ -817,6 +817,13 @@ code_blockt get_user_specified_clinit_body(
           }
         }
       }
+      if(symbol_table.lookup(real_clinit_name) == nullptr)
+      {
+        // Case where there is a user provided clinit, but the real clinit
+        // doesn't appear in the symbol table. This may occur when some class
+        // substitution happened after compilation.
+        return code_blockt{};
+      }
       code_blockt body;
       for(const auto &value_pair : static_field_values)
       {