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: ec868e4ee2 ("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>

Documentation/media/Makefile

@@ -36,7 +36,7 @@ quiet_cmd_genpdf = GENPDF  $2
       cmd_genpdf = convert $2 $3
 
 quiet_cmd_gendot = DOT     $2
-      cmd_gendot = dot -Tsvg $2 > $3
+      cmd_gendot = dot -Tsvg $2 > $3 || { rm -f $3; exit 1; }
 
 %.pdf: %.svg
 	@$(call cmd,genpdf,$<,$@)