localeconv() and setlocale() thread-safety

convert.c has a mechanism to convert the decimal separator in numeric
strings between '.' and the separator of the current locale (e.g. ',' in
Finnish).

The implementation is not thread-safe. If you switch the locale in an
application, and then run a query in two threads concurrently, and they
both call current_numeric_locale() at the same time, they might both try
to free the old current_locale string, causing a crash.

That might seem unlikely to happen in practice, but I just received a
stack trace from a crash dump from a testing environment that contained
words "psqlodbc", "lconv", and "free". The stack trace was missing/had
garbled symbols, so I couldn't make more sense of it, but
current_numeric_locale() is the only place in psqlodbc that deals with
lconv structs. The crash may well have been something completely
unrelated, but this seems worth fixing anyway.

There are more issues that we can't do much about, if an application
changes the locale in one thread, while another thread is running a
query. In that situation, the decimal separator we got from localeconv()
might not match the decimal separator that our earlier call to sprintf()
used. But as I said, there isn't much we can do about that, since there
is no thread-safe version of localeconv().

I'll commit the attached patch that fixes the first problem, unless
someone has a better idea.

PS. Does anyone know an example of a locale that uses a multi-byte
character as the decimal separator?

- Heikki