c++/7400: dynamic_cast segfaults with -shared option

[David Jung]

http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=7400

Also occurs in gcc 3.2-1 (from Redhat null (8.0beta?) ).
Crash is slightly different (tries to access address 0x0).