core: Do not attempt to destroy a default context that doesn't exist
    
    Calling libusb_exit(NULL) when a successful call to libusb_init(NULL)
    has not been made results in a segmentation violation. This is
    definitely a user error, but we can easily guard against it.
    
    Closes #511
    
    Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>
    (cherry picked from commit a5624b22267ec0e146825d3fe94d9e4b2f5ae503)
    
    patch_name: 0009-core-Do-not-attempt-to-destroy-a-default-context-tha.patch
    present_in_specfile: true
    location_in_specfile: 9
    squash_commits: true