fix for arraylength

The type of the array for arraylength doesn't matter -- the previous
implementation uses the first letter of the statement, which only happens to
be a valid type character ('a').

Author: Daniel Kroening

jbmc/src/java_bytecode/java_bytecode_convert_method.cpp

@@ -1603,8 +1603,9 @@ code_blockt java_bytecode_convert_methodt::convert_instructions(
     {
       PRECONDITION(op.size() == 1 && results.size() == 1);
 
+      // any array type is fine here, so we go for a reference array
       dereference_exprt array{
-        typecast_exprt{op[0], java_array_type(statement[0])}};
+        typecast_exprt{op[0], java_array_type('a')}};
       PRECONDITION(array.type().id() == ID_struct_tag);
       array.set(ID_java_member_access, true);