[otlayout] Fix up recent Context matching change

Commit 6b65a76b40522a4f57a6fedcbdfc5a4d736f1d3c.  "end" was becoming
negative.  Was trigerred by Lohit-Kannada 2.5.3 and the sequence:
U+0CB0,U+200D,U+0CBE,U+0CB7,U+0CCD,U+0C9F,U+0CCD,U+0CB0,U+0C97,U+0CB3
Two glyphs were being duplicated.