Strange assertion error after upgrading to gtk+-1.2.9

[Ragnar Ouchterlony <ragnar lysator liu se>]

I compiled the new gtk+-1.2.9 (previous was 1.2.8) and I encountered a
strange assertion when I run some programs (but not all) like the gimp and
Downloader for X (also known as nt):

gimp: wcsrtombs.c:116: __wcsrtombs: Assertion `data.outbuf != (unsigned char *) dst' failed.

It is the same message on all occasions (except for the program name).
This has only occurred since I installed gtk+-1.2.9, it worked fine with
gtk+-1.2.8. I have tested to recompile the nt program but it had the same
problem afterwards, so I don't think the problem is that I need to
recompile my programs.
Note that I also upgraded glib to 1.2.9.

I use glibc-2.1 on a slightly enhanced RH-6.2 system.
I compiled using gcc-2.95.2 and I have two installations: the original
RH-6.2 package and my own gtk+-1.2.9. It worked fine for gtk+-1.2.8 that I
installed previously in the same manner.

Any idea what has gone wrong?

/Ragnar Ouchterlony