Modify wchar conversion routines to not fetch the next byte past the end

Modify wchar conversion routines to not fetch the next byte past the end
of a counted input string.  Marinos Yannikos' recent crash report turns
out to be due to applying pg_ascii2wchar_with_len to a TEXT object that
is smack up against the end of memory.  This is the second just-barely-
reproducible bug report I have seen that traces to some bit of code
fetching one more byte than it is allowed to.  Let's be more careful
out there, boys and girls.
While at it, I changed the code to not risk a similar crash when there
is a truncated multibyte character at the end of an input string.  The
output in this case might not be the most reasonable output possible;
if anyone wants to improve it further, step right up...