Fix maybe-uninitialized warning in py-infthread.c

I got this warning from py-infthread.c using the Fedora 34 system GCC:

../../binutils-gdb/gdb/python/py-infthread.c:102:30: warning: ‘extra_info’ may be used uninitialized in this function [-Wmaybe-uninitialized]

I think this happens because GDB_PY_HANDLE_EXCEPTION expands to an
'if' whose condition is always true -- but GCC can't know this.  This
patch avoids the warning by adding a harmless initialization.