flatpak: Check for an uninitialised remote_name

-Werror=maybe-uninitialized suggested that remote_name could be used uninitialised here. I don’t think it’s possible (there will always be an operation set by flatpak which will cause it to be set), but let’s initialise the variable to NULL and add an assertion to encode our assumptions about it being set, to catch this potential error.

Signed-off-by: Philip Withnall pwithnall@gnome.org