Patch against centericq-4.20.0

Overview
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There is a bug in centericq-4.20.0 that causes a segmentation fault when in a
conference with a null subject (or when recent events in the conference indicate
that a null subject has been set).

Description of Patch
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This bug is on line 1365 of src/hooks/jabberhook.cc
If the subject is null, xmlnode_get_data(x) returns NULL which causes a 
segmentation fault when it is cast to a string.

The fix changes this, such that in the case where NULL is returned, an empty 
string ("") is returned in its place.

Note:  cvs did not build on my machine when I tried (2005-05-30).
Admittedly, I did not try very hard to fix it.  There appeared to be an issue
with generating the configure script using autoconf.