Roboto was hitting this.  FreeType also has pretty much the
same code for this, in ttcmap.c:tt_cmap4_validate():

    /* in certain fonts, the `length' field is invalid and goes */
    /* out of bound.  We try to correct this here...            */
    if ( table + length > valid->limit )
    {
      if ( valid->level >= FT_VALIDATE_TIGHT )
        FT_INVALID_TOO_SHORT;

      length = (FT_UInt)( valid->limit - table );
    }

Can happen either in broken fonts, or as a result of sanitize().

Bug 77800 - cppcheck reports

Apparently they can intermix with other OT_A.

Test: U+0915,A8E2,1CD0

Patch from Martin Hosken.  I expect this to fix the following bugs:

https://bugs.freedesktop.org/show_bug.cgi?id=75076
https://bugzilla.gnome.org/show_bug.cgi?id=723582
https://bugzilla.redhat.com/show_bug.cgi?id=998812

Tests: U+0967,0951 U+0031,093F

Only allow it on DOTTED CIRCLE.  No effect on test numbers.

Test: U+0930,094D,00A0

No functional change intended.

No functional change.

Unicode 6.x IndicSyllableCategory categorizes them as
placeholders, but they can subjoin.

No functional change.

In fact, the previous grammar was ambigious.  No functional
change.

In anticipation of adding more characters to that class of clusters.

It's spacing, but otherwise the same as the other ones.

Up to two of them come after all OT_A characters.

According to Roozbeh, there are valid combinations in Unicode
proposals for up to three.  Previously we were allowing up to two.

Also adjust U+0953,0954 handling.

Sinhala and Telugu use "explicit" reph.  That is, the reph is formed by
a Ra,H,ZWJ sequence.  Previously, upon detecting this sequence, we were
checking checking whether the 'rphf' feature applies to the first two
glyphs of the sequence.  This is how the Microsoft fonts are designed.
However, testing with Noto shows that apparently Uniscribe also forms
the reph if the lookup ligates all three glyphs.  So, try both
sequences.

Doesn't affect test results for Sinhala or Telugu.

https://code.google.com/a/google.com/p/noto-alpha/issues/detail?id=232

Fixes https://github.com/behdad/harfbuzz/pull/40

The grammar in the OT spec, and the existing Windows implementation
seem to be confused around where to allow Asat around the medial
consonants.

The previous grammar for medial group was allowing an Asat after
the medial group only if there was a medial Wa or Ha, but not if
there was only a medial Ya.  This doesn't make sense to me and
sounds reversed, as both medial Wa and Ha are below marks while
Asat is an above mark.  An Asat can come before the medial group
already (in fact, multiple ones can.  Why?!).  The medial Ya
however is a spacing mark and according to Roozbeh it's valid
to want an Asat on the medial Ya instead of the base, so it looks
to me like we want to allow an Asat after the medial group if
there *was* a Ya but not if there wasn't any.  Not wanting to
produce dotted-circle where Windows is not, this commit changes
the grammar to allow one Asat after the medial group no matter
what comes in the group.

Test: U+1002,103A,103B vs U+1002,103B,103A

One of them currently produces dotted-circle.  Fix and detailed
message coming.