Make gdb.selected_thread().inferior return a new reference

Make gdb.selected_thread().inferior return a new reference

thpy_get_inferior function should return a new reference to the
existing inferior object, and therefore should increment its refcount.

Fixed bug looks like this.
If multiple time call gdb.selected_thread ().inferior, gdb throws exception:

(gdb) pi gdb.selected_thread().inferior
<gdb.Inferior object at 0x7f1952bea698>
(gdb) pi gdb.selected_thread().inferior
Python Exception <type 'exceptions.AttributeError'> 'NoneType' object
has no attribute 'inferior':
Error while executing Python code.
(gdb) info threads
  Id   Target Id         Frame
* 1    Thread 0x7f54f0474740 (LWP 584) "mc" 0x00007f54ef055c33 in