Installing any package with current CVS rpm (on Linux, glibc 2.6.1, gcc 4.3) 
results in a segfault.

The segfault occurs in headerGetExtension(), he->t at the time is 
RPM_STRING_TYPE, he->p.str is non-NULL, but invalid (probably uninitialized).
tagName(he->tag) at the time of the crash is Sha1header.

headerGetExtension is called from addTE() when the crash occurs. (addTE is 
called from rpmteNew, which in turn is called from rpmtsAddInstallElement).