Add a similar check in stack_alloc than in FFI::MemoryPointer#initialize

Author: eregon

src/main/ruby/truffleruby/core/truffle/ffi/pointer.rb

@@ -280,7 +280,7 @@ def self.stack_alloc(*args)
                     when Symbol
                       Pointer.find_type_size(type)
                     else
-                      type.size
+                      raise ArgumentError, "incorrect pointer type: #{type.inspect}"
                     end
         length = (type_size * count)
         offsets << [total_length, length]