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.
This also make the function return early when the real clinit is not in
the symbol table, which can avoid some unecessary computation.

Author: romainbrenguier

jbmc/src/java_bytecode/java_static_initializers.cpp

@@ -792,6 +792,14 @@ code_blockt get_user_specified_clinit_body(
     &class_to_declared_symbols_map)
 {
   const irep_idt &real_clinit_name = clinit_function_name(class_id);
+  const auto clinit_func = symbol_table.lookup(real_clinit_name);
+  if(clinit_func == nullptr)
+  {
+    // Case where the real clinit doesn't appear in the symbol table, even
+    // though their is user specifed one. This may occur when some class
+    // substitution happened after compilation.
+    return code_blockt{};
+  }
   const auto class_entry =
     static_values_json.find(id2string(strip_java_namespace_prefix(class_id)));
   if(class_entry != static_values_json.end())
@@ -833,9 +841,7 @@ code_blockt get_user_specified_clinit_body(
       return body;
     }
   }
-  if(const auto clinit_func = symbol_table.lookup(real_clinit_name))
-    return code_blockt{{code_function_callt{clinit_func->symbol_expr()}}};
-  return code_blockt{};
+  return code_blockt{{code_function_callt{clinit_func->symbol_expr()}}};
 }
 
 /// Create static initializer wrappers and possibly user-specified functions for