Avoid integer-overflow in hb_glyph_position_t.y_advance

This patch avoids integer-overflow when hb_glyph_position_t.y_advance is
LONG_MIN.

BUG=630227

Review-Url: https://codereview.chromium.org/2176053002
Cr-Commit-Position: refs/heads/master@{#407744}
1 file changed