libtracker-sparql-backend: Fix memory leak in Connection.get
libtracker-sparql-backend: Fix memory leak in Connection.get

The leak was caused by a bug in valac (fixed in master). This changes
the code to work correctly with valac versions without the fix as well.

Fixes NB#250455.

outofbits/tracker,hoheinzollern/tracker,hoheinzollern/tracker,hoheinzollern/tracker,hoheinzollern/tracker,hoheinzollern/tracker,outofbits/tracker,outofbits/tracker,outofbits/tracker,outofbits/tracker,outofbits/tracker,hoheinzollern/tracker,hoheinzollern/tracker,outofbits/tracker