xsltAddTextString: Check for overflow when merging text nodes.

BUG=676623

Review-Url: https://codereview.chromium.org/2626983002
Cr-Commit-Position: refs/heads/master@{#445987}
(cherry picked from commit 14b7c024aaec338adcbf87cbeee54cf6137d7f8a)

Review-Url: https://codereview.chromium.org/2676223002 .
Cr-Commit-Position: refs/branch-heads/2987@{#318}
Cr-Branched-From: ad51088c0e8776e8dcd963dbe752c4035ba6dab6-refs/heads/master@{#444943}
3 files changed