This introduces no functional changes, but does squash a false positive
warning from `-Wfloat-conversion`:
```
../gio/gapplication.c:462:15: error: implicit conversion turns floating-point number into integer: 'gdouble' (aka 'double') to '_Bool' [-Werror,-Wfloat-conversion]
          if (*(gdouble *) entry->arg_data)
          ~~  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.
```

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