E.g. yue does not have many thousands separators, so the value may be very
large, possibly as much as maxint.

utf8_in2, when working in backward mode, is assuming that we are giving
the address of the last byte of the previous character (see all other
calls to utf8_in2). Otherwise, utf8_in2 returns the size of the current
multibyte character instead of that of the previous multibyte character.

This first reverts "Fix number_buf buffer overflow"
(commit ada93e2db0)

This for loop is apparently actually expected to to skip over NUL
characters.

Fixes #1302

Instead, this limits number processing to 32 digits, as break_numbers does
not support more and would provide bogus result with further digits.

Also fix the signedness of break_numbers so that the 32th bit
actually effectively works.

Otherwise if e.g. only mbrola-fr4 is installed, when the application tries
to load voice en-french, it actually ends up trying to load mb-fr1-en.

This variant sounds consistent and similar to Reed voice, found on Eloquence

Update changelog for mto

Ref #1271

Fixes #1241