Fixed buffer outflow during deserialization of map objects

[rado-h]

The same fix from #111 applied for the deserialization of maps.

[rtheunissen]

Thank you. Could you please explain to me how this allows for buffer overflow?

[rado-h]

Not sure if either overflow or underflow is the most appropriate term here, I just went with the same name as the previous request. What happens is the serialization output from igbinary_serialize doesn't seem to contain any curly brackets so the condition for end might not be met.
For example, while the output of

serialize(new \DS\Map)

would be

C:6:"Ds\Map":0:{}"

the output of

igbinary_serialize(new \DS\Map)

would be

"���Ds\Map�"