doc-rst: Delete output of failed dot-SVG conversion

As we use redirection to create the SVG file, even a failed conversion
will create the file and 'make' will consider it up-to-date if the
build is retried.  We should delete it in case of failure.

Fixes: ec868e4e

 ("docs-rst: media: build SVG from graphviz files")
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>