tools: libxl: free devpath on failure in libxl__blktap_devpath

tools: libxl: free devpath on failure in libxl__blktap_devpath

The underlying code paths in tap_ctl_create attempt to handle both
*devpath == NULL (by allocating) and *devpath != NULL (caller provided
name) and if they allocate tend to write the return immediately before
doing other potentially error generating tasks. All of which makes
handling this at a lower level rather more complicated than handling
it in the error path of libxl__blktap_devpath.

Note that libxl__blktap_devpath initialises devpath to NULL and if the
earlier GCSPRINTF succeeds then the value is returned earlier.
Therefore if we make it to the call to tap_ctl_create then devpath is
still NULL on entry, therefore on the error path devpath is either
still NULL or has been set to a freshly allocated value by
tap_ctl_create. Since free(NULL) is fine it is sufficient to just
free(devpath).

I also considered adding a non-NULL devnull to the gc, even on
failure, but that would have required a comment to explain the
apparently strange behaviour.

Compile tested only.

CID: 1215177

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>