Fix remotely-triggerable crashes by validating strings in a few

Fix remotely-triggerable crashes by validating strings in a few
messages related to buddy list management.  Fixes #14682

I changed the four functions that parse incoming authorization-related
SNACs.  The changes are:

- Make sure we have a buddy name and it is valid UTF-8.  If not, we
  drop the SNAC and log a debug message (we can't do much with an empty,
  invalid or incorrect buddy name).  This wasn't a part of the bug
  report and I doubt it's actually a problem, but it seems like a good
  idea regardless.

- If the incoming message is not valid UTF-8 then use
  purple_utf8_salvage() to replace invalid bytes with question marks.  I
  believe this fixes the bug in question.

--HG--
branch : release-2.x.y
extra : convert_revision : 757272a78a8ca6027d518e614712c3399e34dda3