ui-log: handle parse_commit() errors

If parse_commit() fails, none of the fields in the commit structure will
have been populated so we will dereference NULL when accessing
item->tree.

There isn't much we can do about the error at this point, but if we
return true then we'll try parsing the commit again from print_commit()
and we can report an error to the user at that point.

Coverity-id: 13801
Signed-off-by: John Keeping <john@keeping.me.uk>

ui-log.c

@@ -141,7 +141,9 @@ static int show_commit(struct commit *commit, struct rev_info *revs)
 
 	/* When we get here we have precisely one parent. */
 	parent = parents->item;
-	parse_commit(parent);
+	/* If we can't parse the commit, let print_commit() report an error. */
+	if (parse_commit(parent))
+		return 1;
 
 	files = 0;
 	add_lines = 0;