ticket:10906: prevent SimpleDateFormat from trying to parse when ParsePosition is less than 0

git-svn-id: http://source.icu-project.org/repos/icu/icu@35874 251d0590-4201-4cf1-90de-194747b24ca1
ticket:10046: Andy's review comments.

git-svn-id: http://source.icu-project.org/repos/icu/icu@35876 251d0590-4201-4cf1-90de-194747b24ca1
ticket:10046: Fix comments.

git-svn-id: http://source.icu-project.org/repos/icu/icu@35877 251d0590-4201-4cf1-90de-194747b24ca1