Fix crash in RunInfo::NumGraphemes

Fix crash in NumGraphemes when called with an invalid end char position.

Bug: 913975
Test: fast/text/international/ar_tab_selection_crash.html
Change-Id: I93a94ba04e3e02b10ac8ef4186cf606b7df5c859
Reviewed-on: https://chromium-review.googlesource.com/c/1374630
Commit-Queue: Koji Ishii <kojii@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616145}