Protect against small overread in SASLprep validation

(This is a cherry-pick of 390b3cbbb, which I hadn't realized wasn't
backpatched. It was originally reported to security@ and determined not
to be a vulnerability; thanks to Stanislav Osipov for noticing the
omission in the back branches.)

In case of torn UTF8 in the input data we might end up going
past the end of the string since we don't account for length.
While validation won't be performed on a sequence with a NULL
byte it's better to avoid going past the end to beging with.
Fix by taking the length into consideration.

Reported-by: Stanislav Osipov <[email protected]>
Reviewed-by: Daniel Gustafsson <[email protected]>
Discussion: 
https://postgr.es/m/CAOYmi+mTnmM172g=_+yvc47hzzeasypy2c4uby3hk9p-axn...@mail.gmail.com
Backpatch-through: 14

Branch
------
REL_15_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/ff1d5810e907525b396c67ad900114dd4ef83dc2

Modified Files
--------------
src/common/saslprep.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)

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