This bug doesn't cause a crash on my machine here (Suse 9.2).
Looking at frame 170 in the trace, it looks like
tvb_get_ephemeral_text() struggles with the null character in the middle
of the 4th parameter (in the WWW-Authenticate header) and returns NULL.
The attached patch uses tvb_format_text() instead which also does a
better job of showing the string.